Massachusetts Senate poll loss threatens Obama agenda
Republican Scott Brown's victory speech
Republican Scott Brown has won a shock victory in the race for the US Senate seat in Massachusetts left vacant by Democrat Edward Kennedy's death.
The result is a huge blow to President Barack Obama, whose healthcare reform programme is now in doubt.
Democrat Martha Coakley conceded she had lost the race after early results gave Mr Brown a healthy lead.
The Republican win means the Democrats have lost their filibuster-proof 60-seat majority in the Senate.
This will make it much harder for Mr Obama to pass a healthcare reform bill - the most important domestic policy objective of his first year as president.
The BBC's Paul Adams, in Boston, says Ms Coakley's defeat is a humiliating defeat for the Democrats, and a deeply unwelcome anniversary present for President Obama exactly one year after his inauguration.
This is a referendum on the Barack Obama agenda and a way of working in Washington, an arrogant approach to politics
Sen Robert Menendez, head of the Senate Democrats' campaign committee, said he had "no interest in sugar-coating" the result.
"There is a lot of anxiety in the country right now," he added. "Americans are understandably impatient."
President Obama had campaigned personally on behalf of Ms Coakley.
Analysts say the race should have been an easy win for her in a state which has traditionally voted for Democratic candidates for the US Senate.
Just weeks ago, Ms Coakley, the state attorney general, had a double-digit lead in polls and seemed destined to win.
But a lacklustre campaign allowed her Republican opponent to seize on voter discontent and overtake her in the final stretch.
Voters flocked to the polls through the snow and rain that fell all day on Tuesday.
Ms Coakley said she had received a telephone call from President Obama, who told her: "We can't win them all."
Senior Republican figure Mitt Romney said the vote heralded a political sea-change.
"This is really a referendum on the Barack Obama agenda and I think a way of working in Washington, which is kind of an arrogant approach to politics in this country," the former presidential candidate told Fox News.
Third major loss
White House spokesman Robert Gibbs said President Obama had called Mr Brown to congratulate him and to tell him he was looking forward to working with him.
WHAT NEXT FOR OBAMA AGENDA?
Obama's bid to reform healthcare and pass climate bill now in doubt
On healthcare, Democrats in House could pass bill already passed by Senate
Or rush a compromise bill through before Brown can take his seat
But that could spark a political backlash on Democrats, even if both houses could agree it so soon
On climate, cap-and-trade bill passed by the House faces even more difficulties in Senate
Senate Democratic leader Harry Reid said Mr Brown could take his seat "as soon as the proper paperwork has been received" from Massachusetts officials.
Correspondents say the vote does not bode well for the Democrats ahead of November's congressional elections, and that if they cannot hang on to a party stronghold such as Massachusetts they could be vulnerable almost anywhere.
The result comes amid opinion polls showing nearly half of Americans think President Obama is not delivering on his major campaign promises.
It was the third major loss for Democrats in state-wide elections since he became president: Republicans won governors' seats in Virginia and New Jersey in November.
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