(USA TODAY) -- Former senator Scott Brown will not run for the seat of departingSen. John Kerry in Massachusetts, saying Friday he was unsure "ofreturning 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.

Brownwas defeated in November by Democrat Elizabeth Warren, in the nation'smost expensive Senate race. Democrats had made Brown a top target sincehe was the surprise winner of a 2010 race to replace Democrat EdwardKennedy, who died the previous summer.

Kerry will step downFriday afternoon and take his oath as secretary of State, replacingHillary Rodham Clinton as the nation's top diplomat.

Brownsaid he gave "serious thought" to running, but it would have been histhird 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.

MassachusettsGov. Deval Patrick has already appointed William "Mo" Cowan, a lawyerand his former chief of staff, to be the interim senator until voterspick 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. "Asthe Democratic primary ... turns uglier and nastier each day, theMassachusetts special election provides a real pick-up opportunity forRepublicans, and we intend on defeating whichever career politicianlimps through," said Rob Collins, executive director of the NationalRepublican Senatorial Committee.

Brown was alittle-known state senator when he ran for the U.S. Senate afterKennedy's death. He defeated Democrat Martha Coakley, the state'sattorney general, who ran what many people believed was a lacklustercampaign in 2010.

In his statement, Brown suggestedhe will continue in public service -- but made no mention of whether hewill run for governor next year. Serving in Congress, he said, is "notthe only way for me to advance the ideals and causes that matter most tome."

While Massachusetts consistently votesDemocratic in presidential elections, it doesn't always do the same ingovernor's races. Three Republicans -- William Weld, Paul Cellucci andMitt Romney -- were elected governors from 1990 to 2006.