In a dramatic upset that will end Democrats' super majority in the Senate, Massachusetts Republican Scott Brown defeated Democrat Martha Coakley to claim the seat held by the late Ted Kennedy for more than 46 years.
Coakley called Brown to concede the race shortly before 9:30 p.m., a Democratic aide told ABC News.
After Brown's victory was announced, the throng of supporters at his campaign headquarters chanted "John Kerry's next."
Brown, 50, a lawyer and former model, will be the 41st Republican in the Senate, meaning Democrats will no longer be able to prevent a GOP filibuster -- a major complication for President Obama's agenda and the health care legislation currently pending in Congress.
Obama called Brown to congratulate him on his victory and told him "he looks forward to working with him on the urgent economic challenges facing Massachusetts families and struggling families across our nation," White House press secretary Robert Gibbs said.
Massachusetts Secretary of State William Galvin said he would notify the Senate on Wednesday that Brown had been elected, even though he had said earlier it could take more than two weeks to certify the special election results.
That delay could have given Democrats time to try to push through final passage of Obama's health care plan.
Senate Majority Leader Harry Reid, D-Nev., said tonight that Brown would be seated "as soon as the proper paperwork has been received."
Brown has said he opposes Democrats' health care overhaul working its way through Congress, and some Democrats are even acknowledging Brown's win could imperil passage of other big ticket items, including climate change legislation, regulatory reforms and a banking industry overhaul.
Senate Majority Leader Harry Reid said tonight the loss of the 60-vote majority will not change the Democrats' priorities.
"While Senator-elect Brown's victory changes the political math in the Senate, we remain committed to strengthening our economy, creating good paying jobs and ensuring all Americans can access affordable health care," Reid said in a statement. "We hope that Scott Brown will join us in these efforts. There is much work to do to address the problems Democrats inherited last year, and we plan to move full speed ahead."
However, there has so far been little or no bipartisanship on any of the major issues Congress has faced.
Obama himself acknowledged the stakes in his stump speech for Coakley in Boston on Sunday, saying "a lot of these measures are going to rest on one vote in the U.S. Senate."
With that vote now in Republican hands, Democrats are experiencing growing concern about the future of their majorities in Congress in 2010.
"I think what's happened in the last few weeks here is a wakeup call for the Democrats both here in Massachusetts and around the country," Phil Johnston, a former chairman of the Massachusetts Democratic Party, told ABC's "Top Line" today.
Obama, who has been "both surprised and frustrated" with how hotly contested the Bay State special election became, recognizes the "tremendous amount of upset and anger" among voters about "where we are economically," White House press secretary Robert Gibbs said earlier today.
While the administration continues to deflect suggestions that the Massachusetts race may be a referendum on the Obama presidency, Republicans say the symbolism of Brown's victory is hard to deny.