In January 2010, Brown faced state Attorney General Martha Coakley in a special election called after Kennedy's death the previous year.
With conservatives focusing on a chance to claim the seat Kennedy had held for almost five decades, Brown attacked a favorite conservative target, Obama's health care overhaul, promising to be the pivotal vote to block the plan in a closely divided Senate. However, the Senate approved the bill shortly before the special election, which Brown won.
Brown faced liberal stalwart Elizabeth Warren, a consumer advocate disdained by conservatives, in the 2012 contest for a full term. It became the most expensive Senate contest in the nation. Brown lost to Warren by 8 percentage points.
Intense national interest in both elections helped send lots of money into Brown's campaign treasury. In 2010, more than 60 percent of his contributions came from outside Massachusetts, which was among the highest rates in the nation, according to the Center for Responsive Politics. In the 2012 cycle, as control of the Senate was thought to depend in part on the Massachusetts race, no Senate incumbent raised more out-of-state money than Brown.
Some conservative activists who helped fuel Brown's campaigns have decided that he is not the conservative lawmaker they had hoped he would be.
"He had some bad votes, but he had some good votes," said Tim Phillips, president of Americans for Prosperity, a conservative organization active in electoral politics across the country.
Brown sided with Democrats in supporting Obama's jobs bill and later became one of just three Republicans who voted for the Dodd-Frank law that sought to toughen financial-industry regulations. He also voted for the New START treaty to further limit U.S. and Russian nuclear arsenals.
On one make-or-break issue for tea party activists, Brown remained firm: his opposition to the president's health care overhaul. That issue lost some of its power after its passage by Congress in 2010 and the Supreme Court's decision in June 2012 that it was indeed constitutional.
So far, there's no conservative rallying cry ahead of the 2013 special election. Despite the differences, Republican strategist Ron Kaufman said the race could attract national attention in a year with few high-profile elections, as Brown did in 2010.
"Some things become bigger than they are," Kaufman said.
"It will be different, but I think that there's still an awful lot of people very, very angry right now at Washington," said Kaufman, a Massachusetts' national Republican committeeman. "If they really want to slow down the president, this would be the way to do it."
Walsh, of the state Democratic Party, shot back: "I think that what Ron Kaufman was saying is plainly obvious to us — that Scott Brown's presence in the United States Senate would slow down the president's agenda. And I think the voters of Massachusetts are entire, entirely against that."