NEW YORK (AFP) - Republicans won two key state elections, dealing a stinging blow to President Barack Obama and his Democrats 12 months after they swept to power.

Obama's favored candidates lost battles for the governorships of New Jersey and Virginia in Tuesday's elections but gained a vacant congressional seat in a strongly Republican district of upstate New York that amounted to scant compensation for the president.

Republicans Chris Christie in New Jersey and Virginia's Bob McDonnell unseated Democrats to boost their party still reeling from the scale of Obama's historic White House victory on November 4 last year.

"The Republican Party's overwhelming victory in Virginia is a blow to President Obama and the Democrat Party," Republican National Committee (RNC) chairman Michael Steele said. "It sends a clear signal that voters have had enough of the president's liberal agenda."

Obama spokesman Robert Gibbs played down the wider significance of the three off-year election races.

"I don't think the president is looking at these and believes that they say anything about our future legislative efforts or our future political efforts," Gibbs said.

Exit polls conducted by television networks tilted in favor of his argument, with 55 percent of Virginia voters and 60 percent of New Jersey voters saying Obama did not factor into their decision.

But the devastating losses in both states amounted to a serious wake-up call for the Democrats ahead of crucial mid-term congressional elections next year.

"We had independents and Democrats that came over to support us," McDonnell said in his acceptance speech.

In New Jersey, Christie pulled off an upset victory over Jon Corzine in the heavily Democratic state, with a 49-45 percent margin according to preliminary results.

McDonnell defeated Creigh Deeds by 59-41 percent in Virginia, with almost all precincts reporting.

Virginia helped propel Obama into office a year ago as the nation's first black president, the first time it had backed a Democratic presidential contender in over four decades.

The New Jersey loss was likely harsher for Obama, who campaigned heavily for Corzine in the state long dominated by the Democratic Party.

But Democrats won a consolation prize of sorts with a surprise victory by their candidate in the New York special election after a bitter feud among conservatives that morphed into a battle for the soul of the Republican Party.

Preliminary results gave Democrat Bill Owens 49 percent to 45 percent for his main challenger Doug Hoffman, who ran on the tiny Conservative Party ballot but received high-profile backing from more conservative Republicans like former vice presidential candidate Sarah Palin.

Ahead of the 2010 mid-terms and with Obama bogged down in confrontations over the economy, health care reform and the Afghanistan war, the off-year races were given greater clout.

The Republican Governors Association quickly congratulated McDonnell in Virginia, saying his victory gave the party "tremendous momentum heading into 2010."

If the races showed that Obama's Democratic machine is not invincible, they also bared rifts in the Republican Party over how to rebuild after last year's drubbing in presidential and congressional elections.

Hoffman's storming run made him a standard bearer for the wing of the Republican Party organizing nationwide "tea party" protests against Obama.

Other Republicans argued for a more centrist stand aimed at attracting independent voters. The official Republican candidate, Dede Scozzafava, dropped out of the race over the weekend and endorsed Owens over Hoffman.

Voters also chose mayors in major cities including New York, Atlanta, Pittsburgh, Detroit, Houston and Seattle.

Maine became the latest in a string of states to throw out legislative measures recognizing same-sex marriage, with voters defeating a law granting gay couples the right to marry.

However Washington state voters appeared on the verge of approving the right to civil unions for same-sex couples, with early returns showing a razor-thin margin of 51 percent to 49 percent.

New York's mayor, media tycoon Michael Bloomberg, won a surprisingly tight contest for reelection against Democratic challenger Bill Thompson by 51 percent to 46 percent.

Bloomberg, an independent who ran on the Republican ballot, spent a record amount of his own money during the campaign and had been forecast to win by double digits.