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BOSTON — Gov. Deval Patrick named William Cowan, a lawyer in Boston and a longtime friend, to serve as the interim United States senator until voters chose a successor for John F. Kerry in a special election set for June 25..

Mr. Cowan, who is known as Mo, is a former partner in the politically connected law firm of Mintz Levin, and will become the first African-American to represent Massachusetts in the Senate since Edward Brooke, a Republican, held the seat from 1966 to 1978.

Mr. Patrick, who announced his choice at a news conference at the statehouse, had said he wanted to appoint someone who did not want to run for the seat later because that person would have to run a campaign while learning the ropes in the Senate and would likely not do either job well.

The appointment starts the clock ticking toward the special election. It promises to be another bruising campaign, with two Democratic congressmen likely to face off in a primary in April.

The biggest question remaining in this drawn-out saga, which began last fall when Mr. Kerry's name surfaced as a possible successor to Hillary Rodham Clinton as secretary of state, is whether former Senator Scott Brown will jump in.

Mr. Brown has remained mum on the subject, and his camp has sent few signals about his intentions, although recent stirrings suggest he will get in.

If he wins, he would be in the awkward position of becoming the junior senator to Elizabeth Warren, who beat him in November and has only been in office for a few weeks. It would be unusual for a state to have two senators who had run against each other, especially in race that was so nasty.

The June 25 special election would be his third statewide Senate race in three years. He won the special election in 2010 to finish Senator Edward M. Kennedy's term and lost last year to Ms. Warren. If he won this special election, he would presumably run in 2014 for a full six-year term, making four statewide races in four years.

So far, the only Democratic candidate in the race is Representative Edward J. Markey, 66, a liberal and dean of the state's Congressional delegation. Several Democrats, including Mr. Kerry, have closed ranks behind him in the hopes of discouraging anyone else from getting in and avoiding a bruising primary.

But Representative Stephen F. Lynch, a conservative Democrat from South Boston, is expected to announce Thursday that he will challenge Mr. Markey. The primary would be held April 30.

Recent polling showed that Mr. Brown would easily beat Mr. Markey, who is little known outside of his own district. The poll did not test Mr. Lynch's strength.

Mr. Patrick and Mr. Cowan built up a strong friendship over the years, The Boston Globe reported, as both men rose from difficult childhoods to prominence in Boston and in the state. Mr. Patrick also served as a mentor to Mr. Cowan when both were practicing lawyers.

Mr. Cowan has also mentored many black professionals and has served as a talent scout frequently called upon to help diversify the city's institutions. He helped former Gov. Mitt Romney, who faced criticism for the lack of diversity in his judicial picks, identifying minority lawyers who would make good judges. He recruited black lawyers for the law firm Mintz Levin and for Middlesex District Attorney Gerard T. Leone Jr.