In a major upset, Republican Scott Brown has defeated Democrat Martha Coakley in a US Senate race that leaves Barack Obama's health care overhaul in doubt.
With more than 80 per cent of precincts reporting, Brown was ahead by five percentage points over his Democratic rival, CNN and MSNBC reported.
The lead meant it was impossible for Coakley to make up in the remaining ballots to be counted, and she quickly conceded defeat, congratulating Brown on his win.
"We wish him luck in his public service," Coakley, the state's Attorney General, said.
"I am heartbroken at the result and I know you are also, but I know we will get up tomorrow and continue the fight," she told supporters in Boston, Massachusetts.
Threat to health legislation
Coakley had been favourite to take the late Senator Edward Kennedy's seat - he held it for nearly half a century - and her failure signals big political problems for the president's party.
House, Senate and gubernatorial candidates are up for grabs later this year in the US midterm elections, and if the results in Massachusetts are replicated nationwide, Obama could be in real trouble.
More immediately, Brown will become the 41st Republican in the 100-member Senate, which could allow the Republicans to block the president's health care legislation and the rest of Obama's agenda.
Democrats needed Coakley to win for a 60th vote to thwart Republican procedural manoeuvres to block votes on legislation.
White House spokesman Robert Gibb said President Obama had spoken to Brown to wish him well following his victory, as well as comiserating with Coakley.
"The president congratulated Senator Brown on his victory and a well-run campaign," Gibbs said.
"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."
