Scott Brown, the Republican candidate for Senate, has run an aggressive, surprising campaign in Massachusetts, injecting fear into the Democratic machine over what was expected to be an easy victory.

A charismatic and conservative voice, Mr. Brown has capitalized on voters’ disaffection with the status quo, lashing out against high taxes and government spending. Campaigning as vigorously as he trains for triathalons, Mr. Brown and his physique caused a stir when Cosmopolitan, on its Web site, reprised a 1982 centerfold that featured him nearly naked as America’s Sexiest Man.

Mr. Brown was a law student then, well before he began his political climb from town assessor in Wrentham, Mass., to town selectman, and then, in 1998, to state representative. Mr. Brown has been a state senator since 2004, and in his United States Senate campaign, which he announced in September, he has been especially vocal about health care.

Mr. Brown has promised to be the 41st vote against the health care bill, which could be a damaging blow for the legislation after months of protracted debate. He has said that Massachusetts has already addressed coverage needs in the state’s 2006 legislation, and has criticized a federal bill as too costly.