US President Barack Obama has warned Democrats not to “jam” healthcare reforms through the Senate after a Republican won a seat in Massachusetts.
Mr Obama told ABC News any vote on the issue would have to wait until Scott Brown had taken up his seat, adding: “He has got to part of that process.”
Mr Brown will be Massachusetts’ first Republican senator since 1972.
His victory means the Republicans now have enough votes in the Senate to block the Democrats’ healthcare plans.
Earlier, the president’s senior adviser, David Axelrod, said he would take the defeat into account, but would stick to his agenda.
“We’ll have to think through this next year from the standpoint of tactics but in substance the mission can’t change,” he said.
Asked about healthcare, he added: “It’s not an option simply to walk away from a problem that’s only going to get worse.”
Scott Brown Massachusetts senator-elect
The BBC’s Paul Adams, in Boston, says it is a humiliating defeat for the Democrats, and a deeply unwelcome anniversary present for President Obama exactly one year after his inauguration.
Our correspondent adds that it is one of the biggest political upsets in years – in a seat held for almost half a century by Edward Kennedy, a Democratic Party colossus, who died last year.
Senate Democrats were expected to meet at midday local time (1700 GMT) to decide what to do about healthcare legislation.
Meanwhile their leader in the House of Representatives, Steny Hoyer, said party leaders were trying to determine which parts of the bill could still be passed in the Senate, Reuters news agency said.
He added that another key piece of legislation, the cap-and-trade bill, was not dead, but parts aimed at increasing energy independence were more likely to pass than those aimed at reducing carbon emissions.
Senator-elect Brown told journalists his victory sent the message that “people are tired of business as usual in Washington politics”, and vowed to get to work as soon as possible.
He said he would go to Washington on Thursday with the hope of taking up his seat.
“The campaign is over now, and we have to focus on solving problems,” he said.
Earlier Mr Brown, 50, told NBC’s Today show he did not think the vote was a referendum on President Obama’s first year in power.
“ This is a calamity for the Democrats, all the more on the very day the president has been in power a year ”
He said voters had “enjoyed the message” he pushed while campaigning, including his criticism of Mr Obama’s healthcare plans.
“I just focused on what I did, which is to talk about the issues – terror, taxes and the healthcare plan,” he told NBC.
“We already have 98% of our people insured here already in Massachusetts, so we do not need the plan that’s being pushed upon us,” he added.
But he denied he was intent on derailing the reforms.
“I never said I was going to do everything I can to stop healthcare,” he said.
“I believe everybody should have healthcare, it’s just a question of how we do it.”
Asked for his assessment of the Republican victory a year after taking office, President Obama told ABC: “The same thing that swept Scott Brown into office swept me into office.”
“People are angry and they are frustrated. Not just because what has happened in the last year or two years, but what’s happened over the last eight years.”
Mr Obama said he wanted to make clear that any plans by Democrats for a Senate vote on the reform plan before Mr Scott took up his seat were “off the table”.
“The people of Massachusetts spoke. He has got to be part of that process,” he added.
The Republican beat Democratic rival and Massachusetts attorney general Martha Coakley by 52% to 47%.
Dubbed Senator Beefcake in the US media, Mr Brown is a lawyer and former model who posed almost naked for Cosmopolitan magazine in the 1980s while in law school.
The Republican former governor of Massachusetts, Mitt Romney, told Fox News the vote was a referendum on President Obama, and a verdict on “an arrogant approach to politics in this country”.
The party’s chairman, Michael Steele, said Americans were breathing a “sigh of relief” over healthcare.
“People across the country are saying: ‘Slow it down,'” he said, quoted by the Associated Press.
The Republican win has robbed the Democrats of their filibuster-proof 60-seat majority in the Senate.
After conceding the election in a telephone call to Mr Brown, Ms Coakley told her supporters she was “heartbroken at the result”.
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. Just weeks ago, she 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.
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.