By Scott Malone
BOSTON (Reuters) - Former Republican Senator Scott Brown said on Friday he will not run in a June election to fill the Massachusetts U.S. Senate seat vacated by John Kerry, a move likely to dim Republican chances of picking up the traditionally Democratic-held seat.
Brown, who stunned the liberal establishment of Massachusetts with a win in another special election in 2010, lost his re-election bid last year.
"I was not at all certain that a third Senate campaign in less than four years, and the prospect of returning to a Congress even more partisan than the one I left, was really the best way for me to continue in public service at this time," Brown said in a statement.
"I will not be a candidate for the United States Senate in the upcoming special election."
Brown had been under pressure from local party officials either to launch a formal campaign or to step aside to clear the way for an April 30 primary ahead of the June 25 special election.
Two Democratic Representatives, Edward Markey and Stephen Lynch, have already launched their campaigns to replace Kerry, a Democrat, has left the Senate to become U.S. secretary of state.
REPUBLICAN FIELD UNCLEAR
Observers said it was unclear which Republican would step in now that Brown has decided not to run. The best-known possible candidates include William Weld, a former governor of Massachusetts who served from 1991 through 1997, or Kerry Healey, who served as the state's deputy governor under Mitt Romney.
"The Democrats' chances are excellent at this point, provided Markey is the nominee," said Jeffrey Berry, a professor of political science at Tufts University.
If Lynch were to win the primary, however, "then the race becomes more interesting because it is conceivable that liberal Democrats could sit it out."
Recent polls of likely voters showed Brown with a modest lead over Markey and a strong lead over Lynch in a theoretical election matchup.
"He has basically been campaigning non-stop for three years," Lynch said in a statement referring to Brown. "It's perfectly understandable that he wouldn't want to undertake another campaign. I wish all the best to Scott and his family."
Despite losing Brown, Republicans nevertheless expressed confidence.
"It's time to move forward," said Rob Collins, executive director of the National Republican Senatorial Committee. "The Massachusetts special election provides a real pick-up opportunity for Republicans."
Democrats currently hold 53 U.S. Senate seats to 45 for the Republicans. There are two independent senators.
In 2010 Brown won a special election to fill the seat held by the late Edward Kennedy, becoming Massachusetts's first Republican senator in three decades. He lost his re-election bid to Democrat Elizabeth Warren, a former Harvard professor.
Massachusetts Governor Deval Patrick, a Democrat, on January 30 named his former chief of staff, William Cowan, to hold Kerry's U.S. Senate seat until a successor is picked.
Cowan told reporters he viewed the appointment as temporary and had no plans to run in the special election.
Staying out of the Senate race leaves Brown the option of running for governor of Massachusetts in 2014.
Brown did not comment on that possibility in his statement, but said of the Senate: "I know it's not the only way for me to advance the ideals and causes that matter most to me."
(Reporting by Scott Malone; Editing by Vicki Allen and David Brunnstrom)