US Democrats scrambled to stave off embarrassing defeats in three off-year elections Tuesday that are being seen as a mini-referendum on President Barack Obama's first 12 months in power.

Vice President Joseph Biden was due in upstate New York on Monday to campaign in a dramatic congressional race on behalf of the struggling Democratic candidate.

Obama campaigned Sunday in New Jersey's gubernatorial election, where Democrat Jon Corzine - the incumbent governor in a heavily Democratic state - is neck-and-neck with Republican former prosecutor Chris Christie.

Obama has also campaigned in Virginia's gubernatorial contest, but to little avail, with latest polls showing Republican Bob McDonnell on course to beat Democrat Creigh Deeds by a wide margin.

The races will do little to change the immediate power balance in Washington, where Democrats have dominated since last year's general election. But across-the-board defeat would be seen as a rebuke for Obama exactly a year after his historic victory.

Republicans, looking to escape the political wilderness, smell blood - particularly in New Jersey.

"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.

The races are an opening salvo in the 2010 midterm elections, when the entire House of Representatives, a third of the Senate and two thirds of gubernatorial posts are up for grabs.

New York congressional battle has revealed much about Republican attempts to redefine their party ahead of those crucial elections.

The official Republican candidate, Dede Scozzafava, withdrew over the weekend after being overwhelmed by the rival Conservative candidate, Doug Hoffman.

Hoffman ran to the right of the moderate Scozzafava, winning crucial if controversial backing from senior Republicans such as former vice presidential candidate Sarah Palin.

After withdrawing, Scozzafava called on her supporters to switch allegiance not to Hoffman, but the Democrat Bill Owens.

The internal Republican feud laid bare a debate over whether to move the party to the right, with Palin a potential presidential candidate in 2012, or to the center represented by Scozzafava.

The latest Siena poll showed Hoffman taking a lead of 41-36 against Owens after Scozzafava's departure, but with 18 percent still undecided.

There was more bad news for Democrats in New Jersey, where the unpopular governor has struggled throughout the campaign.

A new Monmouth University/Gannett poll showed Corzine and Christie in a statistical dead heat. Voters previously backing an independent candidate - still with 12 percent support, according to the latest poll - were tending to drift to Christie.

Meanwhile, the Republicans appear to have Virginia sewn up.

A Richmond Times-Dispatch poll released Sunday showed McDonnell leading Deeds by 53-41 percent.

Republican victory there would put a dent in Democrats' hopes of bringing the state into their camp. Obama carried Virginia last year in the presidential election - the first Democratic candidate to do so since 1964.

Also on the ballot Tuesday are mayoral posts in major cities including New York City and Atlanta, as well as a referendum in northeastern Maine on whether or not to permit same-sex marriage.

In New York, Mayor Michael Bloomberg, a Republican media tycoon, 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.