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Washington woke up Wednesday to a new Senate make-up, one featuring Republican Massachusetts Senator-elect Scott Brown, who defeated Democrat Martha Coakley in a victory few thought possible just a month ago.

The race for the U.S. Senate seat formerly held by the late Ted Kennedy is a win that could grind President Obama's agenda to a halt and portend unexpected losses for Democrats in the November midterms.

In his victory speech, Brown declared that he had "defied the odds and the pundits," and said he would try to be a "worthy successor" to Kennedy.

"Tonight, the independent voice of Massachusetts has spoken," Brown said. "This Senate seat belongs to no one person, no one political party. ... This is the people's seat."

With nearly all precincts reporting, returns showed Brown leading Coakley 52-47 percent, by a margin of 120,000 votes. Independent candidate Joseph Kennedy was pulling 1 percent.

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The victory marks a stunning upset in a race thought to be safe for Democrats until Brown's campaign began to surge just weeks ago. Even Brown appeared a bit in shock by his victory. Visibly giddy during his remarks, Brown went script and at one point offering up his daughters to the dating circuit -- and later he earned supporters' laughter by flubbing his campaign pitch line, "I'm Scott Brown. I'm from Wrentham. And I drive a truck."

Brown's victory has powerful ramifications for Obama's agenda. The GOP state senator, once sworn in, will break the Democrats' 60-vote, filibuster-proof majority in Washington. This creates problems for proposed legislation ranging from financial regulatory reform to cap-and-trade.

But most immediately the win sends Democrats into a scramble to pass health care reform before Brown arrives in Washington. Democrats were already weighing options for how to fast-track the bill before polls closed Tuesday.

Brown blasted the health care bill in his victory speech and urged the Senate to seat him as soon as possible. But a schedule for Brown's swearing-in was up in the air on Tuesday night.

"The people of Massachusetts have spoken. We welcome Scott Brown to the Senate and will move to seat him as soon as the proper paperwork has been received," said Majority Leader Harry Reid, D-Nev. Massachusetts Secretary of State William Galvin said he would notify the Senate on Wednesday that Brown had been elected.

Coakley, in her concession speech, said she was "heartbroken" by the result but thanked the Kennedy family for their support in the race and said she respects the voters' choice.

"There will be plenty of Wednesday-morning quarterbacking about what happened, what went right, what went wrong .... We will be honest about the assessment of this race and although I was very disappointed, I always respect the voters' choice," she said.

Brown's margin of victory is significant, making it difficult for any potential challenges to slow down his certification as the winner. The state senator becomes the first Republican to be elected to the Senate from the Bay State since 1972.

Kennedy, who died in August, held the post for 47 years.

"This is a lot different than my victory," former Massachusetts Republican Gov. Mitt Romney told Fox News. "To have a Republican senator, that's unheard of. ... This is monumental. This is epic."

He and other Republicans said the race sends a warning sign to Washington that voters are not happy with Obama's policy decisions.

The White House said Obama has spoken with both candidates. Press Secretary Robert Gibbs said Obama told Brown he "looks forward to working with him on the urgent economic challenges facing Massachusetts families and struggling families across our nation."

Considering how much was on the line, Brown's late-in-the-game surge commanded the attention of the Democratic Party establishment, which dispatched top officials over the past week to try to keep the seat formerly held by Kennedy in Democratic hands. Voter interest in the race for U.S. Senate also seemed high throughout the day. Poll workers reported a steady stream of voters at the ballot box despite the snow.

Massachusetts Secretary of State William Galvin was predicting turnout could be as high as 50 percent.

Brown's campaign marked an upset just by being as competitive as it was against Coakley's.

Democrats outnumber Republicans 3-to-1 in the state -- 37 percent of registered voters are Democrats, 12 percent are Republicans and 51 percent are unaffiliated. Obama won the state by 26 percentage points in the 2008 presidential election.

But Brown pulled far more support across the state Tuesday than Republican presidential candidate John McCain did in 2008.

The campaigns had been inundated with help from outside the state in recent days. Obama and former President Bill Clinton both came to campaign rallies for Coakley, and Obama appeared in a television ad.

The National Republican Senatorial Committee in Washington also "emptied out the building" of staff to send nearly everyone to Massachusetts to help Brown get out the vote. The NRSC reportedly quietly shifted $500,000 to help Brown's campaign in the last two weeks.

Brown's swift rise in the reliably blue state has startled Democrats nationally who are already worried about a backlash in the midterms.

"When there's trouble in Massachusetts, there's trouble everywhere, and they know it," Brown said Tuesday night.

Sen. Robert Menendez, chairman of the Democratic Senatorial Campaign Committee, acknowledged the rough road ahead for the party.

"I have no interest in sugar coating what happened in Massachusetts. There is a lot of anxiety in the country right now," he said in a statement. "In the days ahead, we will sort through the lessons of Massachusetts: the need to redouble our efforts on the economy, the need to show that our commitment to real change is as powerful as it was in 2008, and the reality that we cannot take a single thing for granted and cannot afford even a second of complacency."

Brown will have to run for re-election in November 2012.