AFP - The Republican opposition hoped to draw blood Tuesday as voters went to the polls in off-year contests seen as testing US President Barack Obama's popularity 12 months after his Democratic party swept to power.

Gubernatorial elections in New Jersey and Virginia and an intriguing battle for a New York congressional seat will not change the power balance in Washington, where Obama's Democrats have a hefty majority.

But with midterm elections across the country next year, the three contests were being scrutinized for signs of Obama's resilience in the face of bruising debates on the economy, health care reform, and Afghanistan.

The hottest race was in New Jersey, with Democrat Jon Corzine -- the incumbent governor in a heavily Democratic state -- fighting desperately to avoid defeat by former Republican prosecutor Chris Christie.

Latest polls showed them in a statistical dead heat, after Christie lost an earlier lead. The performance of an independent third candidate could decide the race.

Obama campaigned heavily for Corzine, culminating with a rally on behalf of the flagging candidate on Sunday.

Republicans, who last won New Jersey's governorship in 1997, are gunning for an upset.

"A victory for Republicans in deep blue New Jersey would send shock waves through the country that would be felt right up to the doors of the White House," Republican blog redstate.com said.

Virginia's gubernatorial race appeared poised to go to the Republican candidate, Bob McDonnell, despite Obama's campaigning on behalf of Democrat Creigh Deeds.

The state was captured by Obama in his election last year, the first time a Democratic presidential contender had managed to do so since 1964.

Possibly the most telling contest was in New York state's 23rd congressional district, where a candidate backed by senior conservative Republicans forced the withdrawal of the official, moderate Republican.

The outside candidate, Doug Hoffman, ran with the tiny Conservative Party, but has become a standard bearer for the same wing of the Republican Party organizing nationwide "tea party" protests against Obama.

Hoffman surged in the polls over the weekend, taking a lead over the Democratic candidate, Bill Owens.

His surprise success exposed a strategic divide in the Republican Party, where some favor concentrating on the rightwing base and others a centrist position that might attract independent voters.

Deepening the feud, the official candidate, Dede Scozzafava, endorsed her Democratic opponent Owens, rather than Hoffman, when she withdrew from the race.

Democrats control Congress in Washington, but the national picture is muddier than 12 months ago when Obama led his party to a crushing victory over the Republicans.

A CNN/Opinion Research Corporation poll last month showed 36 percent of Americans with a favorable opinion of the Republican Party to 54 percent unfavorable, while 53 percent took a favorable view of the Democrats and 41 percent unfavorable.

A Gallup poll last week found Obama's job approval ratings down from an average 62 percent in his second quarter in office to 53 percent in the third quarter.

This was one of the largest quarter-to-quarter drops for a first-year president.

Also on the ballot Tuesday are mayoral posts in major cities, including New York City, Atlanta, Houston, Detroit and Pittsburgh, as well as referendums in Maine and Washington state on allowing same-sex marriage.

New York's mayor, the media tycoon Michael Bloomberg, looks likely to win a third term after getting City Council to scrap a mayoral two-term limit and spending a record amount of his own money on the campaign.

Bloomberg is an independent, but running on the Republican ballot. His main challenger, the Democrat Bill Thompson, has received almost no support from the Obama camp.

Latest polls showed Bloomberg with a shrinking, but comfortable lead of 12 points, down from 18 points a week earlier.