Massachusetts state Sen. Scott Brown (R) pulled off one of the biggest political upsets in recent history on Tuesday, defeating state Attorney General Martha Coakley (D) in the hotly contested race to fill the late Sen. Edward Kennedy’s (D) seat.

Coakley called Brown and conceded about 9:15 p.m. With 97 percent of precincts reporting, Brown had 52 percent to Coakley’s 47 percent.

The outcome, unthinkable only weeks ago, has prompted deep soul-searching among Democrats, who assumed control of government in Washington, D.C., with President Barack Obama’s inauguration one year ago.

Not even a last-minute visit from Obama himself could stanch the surge of momentum for Brown in the race’s final weeks.

Brown’s victory will strip Senate Democrats of their 60-seat supermajority when Brown is sworn in, something Republicans will likely try to expedite.

The results of the special election are a stark warning sign for Democrats already nervous about the outlook for the 2010 midterm elections and a huge boost of momentum for Republicans, who immediately deemed it a clear rejection of the Democratic agenda in Washington.

In particular, Brown’s strong showing among independents — a key voting bloc that has largely voted for Democrats over the past two election cycles — signals a shift in the country’s political currents.

“This is a wake-up call for Democrats,— acknowledged an aide at EMILY’s List, one of the organizations working to elect Coakley. The Democratic Party and their allies poured more than $5 million into the race to save Kennedy’s seat.

Democrats have several explanations for how they managed to let the seat slip from their grasp, but it essentially boils down to a poorly run campaign combined with a political environment that was more toxic than anyone anticipated.

The biggest lesson they are drawing for the November midterms is that no candidate can afford to be complacent.

“I think all of us probably shouldn’t have rested on our laurels so much,— the EMILY’s List aide said. Internal polling, he added, showed her up by 20 points just before Christmas.

Coakley opted to take time off around the holidays, a decision that has since been lambasted in the media, as has the fact that she held a fraction of the public events that Brown did over the course of the general election campaign.

Brown started to build momentum, going up with his first advertisement on Dec. 30, a full week before Coakley’s first general election ad. And he held a press conference call on New Year’s Eve touting his fundraising numbers.

A Brown adviser told Roll Call the race seemed to turn two weeks ago, after the New Year’s holiday weekend. She declined to pinpoint a specific trigger for the turnaround, though she did note it came after “Coakley took off six days.—

On the Democratic side, the finger pointing began early on Election Day — never a good sign for a campaign — with the Coakley campaign, led by her Massachusetts adviser and longtime confidant Dennis Newman, fending off the brunt of the accusations.

“It all starts with the campaign and the fact that she ran the wrong general election campaign,— said another Massachusetts-based Democratic strategist whose candidate lost in the special election primary. “I think there’s just no way around that fact.—

“Had she or her campaign taken Brown seriously from the get-go, then [he] would have never gotten the lift that [he] needed to reach critical mass,— the strategist said.

However, the EMILY’s List aide said that while Washington groups are pointing the finger at other people now, everyone was pretty much on the same page back in December.

“It wasn’t until after the holidays that you could really see there were issues,— he said.

The Massachusetts strategist, however, said there were also problems with the campaign’s message. The “obvious path— in the general election campaign should have been to expose Brown’s conservative positions, he said. But by ignoring him, they created an opening for Brown to paint himself as a populist outsider, framing his campaign squarely within the anti-establishment currents now dominating politics.

Folksy television spots produced by Eric Fehrnstrom and the team at Shawmut Group featuring President John F. Kennedy morphing into Brown and Brown driving his pickup truck around Massachusetts shaking hands with voters reinforced that image. Fehrnstrom is a former aide to ex-Massachusetts Gov. Mitt Romney (R).

Brown also turned the campaign focus to health care reform, touting his role as the potential 41st vote against the Democrats’ Senate bill, a piece of legislation that has become unpopular even in liberal-leaning Massachusetts.

Democrats interviewed by Roll Call said Coakley would have been better served by focusing the campaign dialogue on the economy and populist appeals against Wall Street.

But as the EMILY’s List aide acknowledged, “Something bigger was going on.—

The independent vote, which makes up just over half the Massachusetts electorate, is what killed Coakley, he said. And he expressed shock that the heavy spending and outreach to rally support for Coakley in the final week did not boost her with those voters.

“I really thought we’d start to see the numbers move and they just didn’t budge,— he said.

It’s a voting bloc that Democrats have to win in November to defend their majorities in Congress, but finding a way to acknowledge and soothe the seething anger in the electorate is no easy task.

“Our candidates need to realize that these voters, especially independent voters, are frustrated,— the EMILY’s list aide said. Candidates will “need to get a sense of their frustration but make it clear that you feel the frustration, too.—

Democratic Senate candidates running in 2010 said they are well aware of those challenges, regardless of what happened in Massachusetts.

“It’s not a surprise that we’re in a tough political climate, but we knew that before Massachusetts,— said Rose Kapolczynski, Sen. Barbara Boxer’s (D-Calif.) campaign manager. Boxer, she added, “has a campaign rule which is always run like you’re 5 points behind no matter what the polls say today, and we’re following that in this race.—

Three Republicans are vying to take on Boxer in 2010, and they and other GOP challengers have seized on the Massachusetts results as a sign that the party can win in traditionally Democratic states this fall. Independent voters make up the largest voting bloc in California, as they do in Massachusetts,

But Kapolczynski said she hadn’t heard of Democrats in Washington, D.C., or in the California donor community panicking over Massachusetts. “I think they know how hard we’ve been working,— she said.

The Democratic Congressional Campaign Committee, which along with House Democratic leaders mobilized 30 House staffers to campaign in Massachusetts, is also bracing for a political backlash over the result, though they say their House candidates are tested and ready.

One aide said the biggest takeaway from Massachusetts for House Democrats is that “Democrats need to define Republicans before they have the opportunity to define themselves.—

“This guy Scott Brown didn’t run as a Republican. His literature didn’t say he was a Republican. The Republicans’ brand is still in the toilet,— the aide said.

“This is a very tough election cycle and voters are definitely angry,— he said. “What we need to make sure we do is make sure voters are angry at the right people.—