Republican Scott Brown on Tuesday beat Democrat Martha Coakley to win the Massachusetts Senate seat left vacant in August after Sen. Ted Kennedy’s death.

“Tonight the independent majority has delivered a great victory,” Brown said at a raucous victory rally in Boston, declaring “the beginning of an election year filled with many, many surprises” for Democrats.

“They will be challenged again and again across this great land. When there’s trouble in Massachusetts, rest assured there’s trouble everywhere – they know it,” he said.

Indeed, Brown’s victory puts the health-care reform bill championed by Obama and Kennedy in peril, dealing a blow to President Obama’s agenda, and possibly his presidency.

Few thought the race would be competitive just over a week ago. It was a historic election: a nightmare come true for Democrats and a surprisingly sudden rejuvenation for Republicans, coming just 364 days after Obama’s triumphant inauguration in Washington.

“This is epic,” said Mitt Romney, the Republican former governor of Massachusetts.

Sarah Palin, Republican former governor of Alaska, called it “a step to take our country back.”

“It goes without saying that we are disappointed in tonight’s result,” said Tim Kaine, chairman of the Democratic National Committee.

The outcome comes on the heels of Democratic setbacks in November, when they lost the Virginia and New Jersey governor’s races. In both the New Jersey race and the Massachusetts race, Obama campaigned for the Democratic candidate, to no avail.

Attention turned immediately to whether Democrats at the state and national level would try to delay Brown from being seated to give Democrats in Congress

But Senate Democrats began to distance themselves from such an action within an hour of the results.

“It would only be fair and prudent that we suspend further votes on health care legislation until Senator-elect Brown is seated,” said Sen. Jim Webb, Virginia Democrat.

Brown said he had called interim Sen. Paul Kirk, a Democrat, and that Kirk “has completed his work as a senator.”

“I’m ready to go to Washington without delay,” Brown said, promising to continue to fight against the Democratic healthcare bill, as he promised during the campaign.

“People do not want the trillion dollar health care plan that is being forced on the American people,” Brown said.

The outcome sets off a furious effort by the White House and congressional Democrats to argue that Brown’s win was not a referendum on health care or on the president’s agenda. They will face an uphill battle both in winning that debate in the public and in persuading wavering Democrats in Congress to vote for any health-care bill.

“This was a repudiation of Barack Obama,” wrote Tom Jensen, with Public Policy Polling. “Certainly Martha Coakley was a bad candidate and ran a terrible campaign but that doesn’t change the fact that we found Obama’s approval rating at only 44% with the electorate for today’s contest, a huge drop from the 62% of the vote he won in the state in 2008.”

Romney, who is considered a Republican frontrunner to challenge Obama for the presidency in 2012, agreed.

“This is really … a referendum on the Barack Obama agenda,” he said, saying the win was a rejection of Obama’s “arrogant approach to politics … shutting out one party.”

“You can’t underscore how significant this victory is,” said Romney, labeling the Obama administration “neomonarchists in Washington who think that government is smarter than the people.”

Sen. Robert Menendez, New Jersey Democrat and chairman of the Democratic Senatorial Campaign Committee, said the “special election in Massachusetts was just that: a special election, with a whole host of circumstances that are unique.”

“I would caution against taking a single unique election and extrapolating what it means for the midterms 10 months away,” Menendez said.

But Menendez admitted that the Democratic party needed to draw some lessons from the stunning defeat.

“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,” he said.

Rep. Chris Van Hollen, chairman of the Democratic Congressional Campaign Committee, blamed the voters’ anger on former President George W. Bush, who he said “drove our economy into a ditch and tried to run away from the accident.”

The president called both Brown and Coakley not long after the results were announced, the White House said.

“The president congratulated Senator Brown on his victory and a well-run campaign. The president told Senator Brown that he looks forward to working with him on the urgent economic challenges facing Massachusetts families and struggling families across our nation,” said White House press secretary Robert Gibbs in an e-mailed statement.

Gibbs said that Obama “thanked Attorney General Coakley for her hard work and urged her to continue her advocacy on behalf of working people.”