White House hopefuls are jetting across the US in last-minute campaigning ahead of the biggest single day of voting ever in a presidential nominating race.
Mr Obama told crowds that sometimes the underdog wins
A total of 24 states will hold nominating contests on Super Tuesday.
The Republican race, in which John McCain appears to have a clear lead in polls over his main challenger, Mitt Romney, may be decided on Tuesday.
But the Democratic race between Senators Hillary Clinton and Barack Obama is very tight and may go further.
Super Tuesday states account for over half the delegates who will go to party conventions to formally choose the nominees to run in November's election.
Mrs Clinton, who began the day with a visit to a child study centre at Yale University where she volunteered as a law student, was later expected to make stops in Massachusetts and New York.
Mr Obama flew out of Chicago and landed in New Jersey, where he attended a rally in East Rutherford.
Introduced by actor Robert DeNiro, he told the crowd, "Sometimes the underdog pulls it out. You can't always believe the pundits and prognosticators".
Republican front-runner John McCain kicked off his final push with a rally in rival Mitt Romney's home-state of Massachusetts, before heading for New Jersey and then on to New York where he is expected to appear at Manhattan's Grand Central Station.
He told reporters in Boston that he was "guardedly optimistic" ahead of the contests.
"We think we can win, but that's why we're campaigning hard right up literally until the polls close," he said.
Pancake pit stop
Mr Romney has an exhausting schedule of events slated including stops in Tennessee, Georgia, Oklahoma and California.
From Nashville, where he enjoyed a pancake breakfast with voters, he underlined his credentials as the most conservative of the Republican candidates.
"If I win California that means you're going to have a conservative in the White House," he said.
Of the 2,025 delegates needed to secure the nomination as the Democratic Party's presidential candidate, California carries 370.
A national poll for the Washington Post and ABC showed Senator Clinton's lead over Senator Obama had narrowed to 4%, while other polls showed the two neck-and-neck in the key state of California.
The same national poll showed Senator McCain well ahead of all his rivals. The Arizona senator had 48% against Mitt Romney's 24%, with Mike Huckabee and Ron Paul trailing far behind.
A Reuters/C-Span/Zogby poll gave Mr McCain double digit leads over Mr Romney in New York, New Jersey and Missouri, although the former Massachusetts governor was ahead 40% to 32% in California.
Contenders used the Sunday talk shows to try push their message.
Speaking on ABC's This week, Mrs Clinton said she had been "taking the incoming fire from Republicans" for some 16 years.
"I'm still here, because I have been vetted, I have been tested," she said, pointing out that Mr Obama "did not face anyone who ran attack ads against him" when he stood for election as senator for Illinois.
But Mr Obama countered on CBS's Face the Nation that Republicans consider Mrs Clinton a "polarising figure".
He said he thought he could win votes that the former first lady would not be able to.
Meanwhile, Mr Romney told ABC that Mr McCain "doesn't understand the economy", and said that under him, the Republicans would be "indistinguishable" from Mrs Clinton on issues such as illegal immigration.
Mr McCain said he was pleased to be "gathering support from all parts of the party", claiming to be "far more conservative" than Mr Romney and pointing to prominent conservatives who had endorsed him.
With the possibility that Mr Romney and Mr Huckabee will split the votes of more conservative Republicans, Mr Huckabee told CNN that Mr Romney should drop out.
"If he wants to call it a two-man race, fine. But that makes it John McCain and me," said Mr Huckabee.