Now Brown is considering whether to seize a chance to return to the Senate — in yet another special election — to take the place of Democratic Sen. John Kerry if he is confirmed as secretary of state. Democrats will be more than ready for Brown this time if he does run.
"The atmosphere would be completely different," said the state Democratic Party chairman, John Walsh. He acknowledged making "unforgivable mistakes" by taking for granted the race in which Brown won the late Sen. Edward M. Kennedy's seat.
"We are not asleep at the switch anymore," Walsh said.
As the Democratic machine begins to stir, national conservative groups active in Brown's first run say they've yet to focus on a Massachusetts Senate election that could be five months away.
"It's a different race," says Amy Kremer, national chairman of the Tea Party Express, which funneled a ton of volunteers and more than $340,000 into Brown's 2010 bid.
"Conservatives in Massachusetts, I'm sure, are excited and want him, but it's definitely not something that people are focused on across the country," Kremer said. "What happened with him in 2010 was it became a nationalized race and people got excited. But right now, it's not on anybody's radar."
Republicans familiar with Brown's thinking expect him to run but say that his candidacy is by no means assured. These Republicans spoke on the condition of anonymity because they were not authorized to disclose information about Brown's decision-making.
Republicans in Washington see Brown as a chance to take a seat from Democrats, who hold a 55-45 edge in the Senate.
The date of the special election won't be announced until Kerry resigns upon his confirmation as secretary of state. State officials expect the special election as early as June.
Despite the time frame, Brown is in no hurry to make his intentions public, according to his Republican allies. For now, his camp is content to let speculation about his candidacy fuel steady news coverage. Since leaving office Jan. 3, Brown has conferred quietly with Republican operatives Eric Fehrnstrom and Peter Flaherty, top aides to former GOP presidential candidate Mitt Romney.
Brown's allies say he is encouraged that a member of the Kennedy family will not run for the seat. Kennedy's widow, Vicki Kennedy, and Edward Kennedy Jr. have said they would not enter the contest.
Kerry and Democrats in Washington are backing longtime U.S. Rep. Ed Markey, the only announced candidate, although Markey's popularity outside his north-central Massachusetts district is unclear. First elected in 1976, he is already drawing criticism from Massachusetts' small Republican class.
"Ed Markey is an uninspiring, unaccomplished political hack," said Massachusetts-based Republican strategist Ryan Williams, a former Romney aide.
If Brown declines to run, there are other possible Republican candidates in the wings, including former Gov. William Weld, former gubernatorial candidate Charles Baker and recent congressional candidate Richard Tisei.
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. But in a year with few high-profile elections, Republican strategist Ron Kaufman said, "some things become bigger than they are."
"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, Massachusetts' national Republican committeeman. "If they really want to slow down the president, this would be the way to do it."