But when the ballot counting began this week, Uhuru Kenyatta, the son of Kenya's first president, surged ahead in the race for president and stayed out front as the margin narrowed early on Friday. Soon, the Obama administration and its allies could face a tough choice, made even more complicated by the appearance of taking sides against a candidate who may very well win.

Does the United States put a premium on its commitment to justice and ending impunity — as it has emphasized across the continent — and distance itself from Mr. Kenyatta should he clinch this election?

Or would that put at risk all the other strategic American interests vested in Kenya, a vital ally in a volatile region and a crucial hub for everything from billion-dollar health programs and American corporations to spying on agents of Al Qaeda?

Even the little things could be tricky. Are the American diplomats who interact with the Kenyan government on a daily basis not supposed to shake Mr. Kenyatta's hand? What about sharing a dais with him? The British have already publicly stated that they would avoid any contact unless it was essential.

"This is going to pose a very awkward situation," said Jendayi Frazer, a former assistant secretary of state for African affairs. "Kenyatta knows he needs the United States, and the United States knows it needs Kenya."

American officials have declined to discuss publicly what a Kenyatta victory would mean, and several reiterated the rather anodyne video message from President Obama in February, in which he said, "The choice of who will lead Kenya is up to the Kenyan people."

But Johnnie Carson, the top administration official for Africa, was not quite so diplomatic when he repeatedly warned soon after that "choices have consequences," which critics say backfired by energizing supporters of Mr. Kenyatta and his running mate, William Ruto, driving many to the polls to rally behind them.

"When you inject yourself into an election," Ms. Frazer said, "you never know how it will play."

Mr. Carson responded, "One comment does not swing a contest."

If he wins the presidency, Mr. Kenyatta, who was leading with about 48 percent of the vote early Friday morning, would become the second African head of state after Sudan's Omar Hassan al-Bashir to face grave charges at the International Criminal Court at The Hague. But that does not mean he will meet the same diplomatic isolation as Mr. Bashir, who is wanted on an arrest warrant and cannot travel to much of the world.

For starters, Mr. Bashir has refused to appear at the court, while Mr. Kenyatta has traveled there to defend himself, so no warrant has been issued. Beyond that, the United States and Sudan were hardly allies when Mr. Bashir was accused of fomenting genocide in Darfur. The relationship was already sour, with Sudan squeezed by sanctions for playing host to Osama bin Laden, among other things.

By contrast, the American-Kenyan partnership has been a particularly symbiotic one, especially recently. American intelligence agents work closely with their Kenyan counterparts, hunting down Qaeda cells in Kenya and Somalia. Kenya receives nearly $1 billion in American aid each year and has agreed to accept captured Somali pirates and hundreds of thousands of refugees, at the request of donors like the United States.

Nairobi, Kenya's capital, is home to the largest American Embassy in sub-Saharan Africa and a sprawling United Nations campus that runs programs across the world, making it especially difficult for the United States to take its resources somewhere else.

"There is really very little leverage that the U.S. and other countries can exercise," said J. Peter Pham, director of the Michael S. Ansari Africa Center in Washington.

One former American official with extensive experience in Africa was more blunt. "We need Kenya more than Kenya needs us," he said.

The United States has to be careful how it handles the Kenyatta issue, analysts say, because Mr. Kenyatta could easily turn to China, which has made important inroads here, building highways and even covertly financing some Kenyan military operations.

Already, the Western concerns about Mr. Kenyatta's candidacy seem to be provoking a backlash.

On Wednesday, the Kenyatta campaign accused the British high commissioner here of "shadowy, suspicious and rather animated involvement" in Kenya's election, a claim the British dismissed as "entirely false."

Some diplomats have also spoken of what they call a "Mugabe factor," warning that Kenya's leaders, if put under too much pressure, could become isolated and testy like President Robert Mugabe of Zimbabwe.

Court documents paint a disturbing picture of Mr. Kenyatta. According to prosecutors, as hundreds of his fellow Kikuyus were being slaughtered by rival ethnic groups in the explosion of violence after the last major election in 2007, Mr. Kenyatta organized meetings with a nefarious Nairobi street gang to take revenge.

Mr. Kenyatta, the documents say, "contributed money towards the retaliatory attack" and was "aware of the widespread and systematic nature of the attack." The outlawed gang, the Mungiki, killed scores of people, including small children burned to death while huddling in their homes.

Mr. Kenyatta has denied the charges and says he will clear his name. Many analysts say the case is rather weak. The trial was supposed to start next month but has been postponed until July.

The case against Mr. Ruto, a charismatic politician with an intensely loyal following, is believed to be stronger. Prosecutors say Mr. Ruto and his inner circle "created a network of perpetrators" and paid them to kill.

There are few figures in the Kenyan political landscape as polarizing as Mr. Kenyatta, 51, a deputy prime minister. Many members of other ethnic groups accuse him and his family of stealing their ancestral land, which became the basis for the Kenyatta family fortune.

But among many Kikuyus, Mr. Kenyatta is seen as a savior. He is a confident speaker, educated at Amherst College and respected in Nairobi's business circles. During the last election crisis, several supporters said, he paid for buses to ferry Kikuyus out of danger zones and bought sacks of food for families who had been burned out of their homes.

"When we needed it, Uhuru was there," said David Wanjohi Chege, a trader.

Many Western governments would clearly prefer the second-place candidate, Prime Minister Raila Odinga, who was trailing with 46 percent of the vote early Friday amid complaints from his camp that some results had been "doctored." The race seemed headed for a runoff.

Diplomats in Nairobi said they do not know what their governments will do if Mr. Kenyatta ultimately wins — though most expected little or no change in policy unless he is convicted or stops cooperating with the court. If that happens, some diplomats spoke of targeted sanctions without wanting to be more specific.

"To be honest, there are so many different scenarios, nobody really knows what we're going to do," one American official said.

The United States has opted not to participate in the International Criminal Court, though the administration has expressed support for "the I.C.C.'s prosecution of those cases that advance U.S. interests and values." Many Kenyans see this as hypocritical.

Mr. Kenyatta's lawyers could probably drag out the trial for years. The only conviction the court has secured, against a Congolese warlord, came six years after he arrived at the court.

Some Kenyans have worried that Mr. Kenyatta, if he wins, may try to pull Kenya out of the court to evade trial. But according to legal scholars, that would not change Kenya's obligations.

"The I.C.C. was definitely a factor in this election, but not necessarily the factor you would expect," said Maina Kiai, a prominent Kenyan human rights defender. "It got people out. People were saying, 'They're our boys, they're our sons, we need to protect them.' "

Bill Brandt: Shadow and Light Nearly 160 prints and related material are at MoMA.

"Bill Brandt: Shadow and Light" at the Museum of Modern Art is the latest chance to experience the several varieties of Brandtian wonder and defamiliarizing strangeness. Organized with sharp acumen by Sarah Hermanson Meister, a MoMA photography curator, it presents nearly 160 vintage prints and related material, including examples of the illustrated magazines that published Brandt's work starting in the mid-1930s. All told the show provides a sweeping view of Brandt's sensibility, his wide-ranging subject matter, his obsession with the printing process as an essential part of the photographer's art and his ability to function creatively even when on assignment.

Following the trajectory suggested by its title, the show begins with the often penumbral images — not unlike Seurat's charcoal drawings — of British life that he took in the 1930s and during World War II. It ends with his Surrealist, relatively light-bathed images of female nudes from the 1950s and '60s, in which exaggerated viewpoints render bodies, and parts of bodies, nearly unrecognizable if not abstract, like smooth biomorphic sculptures worthy of Brancusi or Arp.

The achievements of Brandt (1904-83) are not as firmly fixed in the mind as those of his contemporaries and artistic equals: Walker Evans (1903-75) is known for the consummate lucidity of his images of sharecroppers and weathered Southern architecture. Henri Cartier-Bresson (1908-2004), master of the hand-held Leica, captured people on the fly in elegantly composed "decisive moments."

Brandt was interested in neither. His pictures can be so dark that it takes a while to know what's what, and they tend toward stillness. (He worked for the most part with a Rolleiflex box camera on a tripod.) More than a visual style, his photographs have a kind of atmosphere, an emotional depth, a sense of human vulnerability that extends even to the city views and landscapes, and that expands in several directions as you study the pictures.

Some, like those of London parlormaids on duty in starchy white aprons over black dresses, seem quintessentially of the moment, made for black-and-white film and rife with social tension. Others, though hardly without such commentary, oscillate stylistically in time. A shadowy image of a girl scrubbing a doorsill variously brings to mind 19th-century photography, Chardin and Rembrandt.

Brandt was not beyond ambiguity himself. His desire to be seen as an English photographer recording his homeland led him to obscure the fact that it was an adopted one. When he died in 1983 more than one obituary said he was born in England, though he was actually born in Hamburg to a family of wealthy German bankers and shippers. He suffered from tuberculosis as a teenager, prompting his parents to send him to Swiss sanitariums where enforced rest enabled him to dabble in photography.

In 1927 he went to Vienna, seeking a psychoanalytic cure for his disease and ended up working as a photographer's assistant. He had found his art, although it wasn't yet clear where he would pursue it.

In 1929 he settled in Paris for five crucial years, making periodic trips through Europe with a camera and friends. (His early efforts can be viewed in photo albums kept by his companions, Lyena Barjansky and Eva Boros; Boros later became his wife.) He worked briefly as an assistant to the American photographer, painter and Surrealist Man Ray, taking to heart both his sensibility and his freewheeling darkroom techniques. Brandt familiarized himself with Eugène Atget's monumental ode to Paris and was deeply influenced by Brassaï, the Hungarian-born photographer whose 1933 book, "Paris by Night," as one critic put it, portrayed the "late-night human comedy with a technical and documentary precision unknown before his time."

By 1934 Brandt was in England to stay, driven there perhaps, Ms. Meister suggests, because Paris was nearly overrun with talented photographers. He soon established himself with two books, "The English at Home" (1936) and, with a nod to Brassai "A Night in London" (1938).

Divided into six sections, the MoMA show gives an excellent account of the sheer variety of Brandt's subject matter. He portrayed distinctly different levels of the British class system: from grave, besooted Northumbrian miners to those uniformed parlormaids to wealthy Londoners playing backgammon in a Mayfair drawing room. He roamed London's streets and parks and, during the Blitz, the crowded underground stations where people sought shelter, sleeping cheek to jowl, and where he made some of his best-known images. The London blackout played to his love of darkness. He also explored the gritty industrial towns in the north of England, like Halifax and Jarrow, shrouded in Dickensian gloom, and he took to the open countryside to capture images of Hadrian's Wall, Stonehenge and a ruined farmhouse on the edge of a windswept moor that is thought to have figured in Emily Brontë's "Wuthering Heights."

He made portraits of sculptors, actors and literary notables, and in 1960 zeroed in on the eyes — sometimes the left, sometimes the right — of distinguished, rather ancient artists (among them Jean Dubuffet, Louise Nevelson, Jean Arp and Alberto Giacometti), resulting in extreme close-ups of organs of sight surrounded by leathery, sagging skin that are beyond gender, race and even species. And finally there are the so-called nudes, the surprise ending of an already illustrious career and potent latecomers to the history of Surrealism.

Considered his greatest achievement, these are perhaps the most mysterious of all Brandt's work. In some, like "Belgravia, London" — in which a woman's legs loom up from the bottom of the image, and a foot hooked over the other knee presents a drastically elongated calf — the picture might almost have been taken by the woman herself. (We certainly wonder: Where's Bill?) Others, especially those taken on pebbly beaches in England and France, can seem disturbingly strange. Ears and forearms become colossal. A shockingly odd chunk of flesh resembles an amputated limb but is actually the sole of a bare foot whose bent toes are out of sight.

Both the show and catalog demonstrate the single-mindedness of Brandt's artistry. Many of his seemingly documentary images were posed. His wealthier subjects were often family members or friends; Pratt, the dark-haired parlormaid he frequently photographed, worked for his uncle. When shooting on assignment he tended to retain the best images for himself: the portrait of Robert Graves that appeared in a 1941 issue of Picture Post shows a young man focused on his work; the one from the same session that Brandt held back shows the poet looking more mature, a quill pen clasped between his teeth and a slightly mad, oracular gleam in his eye.

A short, fascinating, well-illustrated catalog essay by Lee Ann Daffner, the Modern's conservator of photography, delves into Brandt's extensive fine-tuning of his negatives to achieve the effects he wanted: highlighting lines and textures with razor blades or pencils; removing imperfections and intensifying lights and darks with ink, wash or white gouache (as on the white of Dubuffet's eye).

In the darkroom Brandt manipulated his negatives to such an extent that each print was more or less unique. And to the dismay of his admirers in the 1960s he began to reprint his early images, replacing their soft seeping grays and blacks with stark blacks and whites, perhaps as a result of the sharper, lighter effects of the nudes. Ms. Meister might have included a few examples of these, along with a few examples of his nudes from the late 1970s, which she characterizes as disturbing and implicitly violent.

At the end of her excellent catalog essay she notes that she has also omitted the color landscapes and the three-dimensional assemblages Brandt made from found objects near the end of his life that she describes as "a private curiosity." She concludes with the slightly prim observation that "like most great artists, Brandt, upon occasion, needed an editor."

Learning of these omissions slightly undermines her beautiful show, revealing it as something of a Brandtian fiction in itself. It gives us Brandt's artistry at full- strength — which may have been needed — without quite revealing all the strange places to which it drove him.