President Obama lent his support to Democrat Jon Corzine in New Jersey
Voters in the US states of New Jersey and Virginia are heading to the polls to choose new governors.
The results will be closely watched by Republicans and Democrats who see it as a test of their parties' standing one year after President Obama was elected.
Two Congressional seats are also up for grabs in New York and California. New York is among cities choosing a mayor.
While local issues have been paramount in these races, a win could boost party morale ahead of 2010 mid-term ballots.
In Virginia, Democratic party candidate Creigh Deeds is battling a former attorney general, Republican Bob McDonnell, for the post of governor.
Voters there surprised some analysts last year when they chose Barack Obama in the presidential election, making him the first Democratic candidate to carry the state since 1964.
But according to a poll by The Richmond Times-Dispatch, Bob McDonnell is favoured by 53% of voters, while Creigh Deeds is preferred by 41%, with 6% undecided.
New Jersey, meanwhile, has witnessed a particularly bitter campaign, with the incumbent Democratic Governor Jon Corzine repeatedly attacking his Republican rival Chris Christie not only over his ties to former President George W Bush and his views on abortion and healthcare, but also poking fun at his weight.
Polling suggests the two candidates are neck and neck.
Lines represent aggregate of polls collated by polling resource site Pollster.com
A survey conducted by Monmouth University and Gannett over the final weekend of the campaign gave Jon Corzine a slim 43% to 41% lead over Mr Christie, with the independent candidate, Chris Daggett, dropping to 8% of the vote.
A defeat in New Jersey would sting the Democrats, especially after Mr Obama threw his weight behind Governor Corzine, repeatedly travelling to New Jersey to back the former Goldman Sachs chief executive on the campaign trail.
"We will not lose this election if all of you are as committed as you were last year," Mr Obama told voters in New Jersey, referring to his own success in mobilising a wide array of voters including Hispanics, African-Americans and the young in the 2008 presidential election.
However, the president's former campaign manager, David Plouffe, warned against reading too much into Tuesday's contests.
"The results of these elections tend to be over-read," he told the NBC network's Today programme. "These are local races. There's 18,000 lifetimes between now and next November."
But it is not just the Democratic Party whose performance will be under scrutiny.
The race to fill a vacant Congressional seat in New York's 23rd district has highlighted a split within the Republican Party, with some choosing to back the tiny Conservative Party's Doug Hoffman rather than endorse Dierdre Scozzafava, the moderate candidate chosen by the Republicans.
Michael Bloomberg is running against Democrat Bill Thompson in New York
The row led Ms Scozzafava - who supports abortion rights and gay marriage - to drop out of the race on Saturday and throw her weight behind the Democratic candidate, Bill Owens.
Also in New York, independent Mayor Michael Bloomberg is seeking a third term, with pre-vote polling putting him well ahead of Democratic challenger Bill Thompson.
Analysts estimate the billionaire businessman could have spent more than $100m (£61m) of his personal fortune on his re-election campaign.
Mayoral elections are also being held in Atlanta, Houston, Boston, Detroit and Pittsburgh.
Meanwhile in Maine, a referendum is being held on whether to accept or reject a same-sex marriage law passed by state lawmakers earlier this year.
If voters pass the measure, Maine will be the first US state to approve gay marriage at the ballot box.
Whatever the results of Tuesday's elections, they will be pored over by the party faithful for clues as to how their candidates may fare in the crucial 2010 mid-term elections, correspondents say.
Next year, the entire House of Representatives, about a third of the Senate and two-thirds of governors' posts will be up for grabs.