Ex-senator Scott Brown won't run for Kerry seat

• A special election will be held June 25 for the rest of John Kerry%27s term
• Brown won a 2010 election to succeed Democrat Edward Kennedy
• The moderate Republican criticized partisanship in Congress

Former senator Scott Brown will not run for the Senate seat vacated by John Kerry in Massachusetts, saying Friday he was unsure "of returning to a Congress even more partisan than the one I left."

The decision by Brown, a moderate Republican, means Democrats now have a better chance of keeping the Massachusetts seat.

Brown was defeated in November by Democrat Elizabeth Warren, in the nation's most expensive Senate race. Democrats had made Brown a top target since he was the surprise winner of a 2010 race to replace Democrat Edward Kennedy, who died the previous summer.

Kerry officially stepped down Friday afternoon and was sworn in as secretary of State, replacing Hillary Rodham Clinton as the nation's top diplomat.

Brown said he gave "serious thought" to running, but it would have been his third Senate race in four years. If he won the June 25 special election, Brown would have to run again in 2014 for his own term.

Massachusetts Gov. Deval Patrick has already appointed William "Mo" Cowan, a lawyer and his former chief of staff, to be the interim senator until voters pick a new senator. Primaries will be held April 30.

Democratic Reps. Ed Markey and Stephen Lynch have already declared their Senate candidacies.

The Senate GOP campaign committee vowed Republicans will be competitive. "As the Democratic primary ... turns uglier and nastier each day, the Massachusetts special election provides a real pickup opportunity for Republicans, and we intend on defeating whichever career politician limps through," said Rob Collins, executive director of the National Republican Senatorial Committee.

Brown was a little-known state senator when he ran for the U.S. Senate after Kennedy's death. He defeated Democrat Martha Coakley, the state's attorney general, who ran what many people believed was a lackluster campaign in 2010.

In his statement, Brown suggested he will continue in public service — but made no mention of whether he will run for governor next year. Serving in Congress, he said, is "not the only way for me to advance the ideals and causes that matter most to me."

Although Massachusetts consistently votes Democratic in presidential elections, it doesn't always do the same in governor's races. Three Republicans — William Weld, Paul Cellucci and Mitt Romney — were elected governors from 1990 to 2006.