Democrats Hillary Clinton and Barack Obama were in a virtual dead heat two days before the biggest round of presidential voting so far while John McCain tried to nail down the Republican nomination for the White House.

With 24 states holding nominating contests from coast to coast on Tuesday, the candidates crisscrossed the country, leading rallies and urging supporters to get out and vote.

The Democratic race narrowed to almost a draw in national polls while McCain hoped to win enough delegates to the Republican convention to be the party's presidential nominee in the November presidential election.

Obama held a slight lead in California, the biggest prize of all where Clinton once led, and was virtually tied with her in New Jersey and Missouri -- three of the states voting on "Super Tuesday" -- in a Reuters/C-SPAN/Zogby poll released Sunday.

Obama picked up a high-profile endorsement in Los Angeles when Maria Shriver announced her support for him -- a few days after her husband, California Gov. Arnold Schwarzenegger, endorsed McCain. Shriver's uncle, Sen. Edward M. Kennedy, and cousin Caroline Kennedy, the daughter of former President John F. Kennedy, announced their support for Obama last week.

"I thought, if Barack Obama were a state, he'd be California," Shriver said. "... Diverse. Open. Smart. Independent. Bucks tradition. Innovative. Inspiring. Dreamer. Leader."

While Obama and Clinton vied to win the most delegates, they also were making the argument of being the most electable candidate to face McCain in November.

Clinton, a target of conservatives since being first lady in the 1990s, said her record was well known and she had already weathered heated attacks while Obama, a first-term senator from Illinois, was still an unknown quantity.

"My opponent hasn't had to go through that kind of baptism by fire," the New York senator told a St. Louis rally. "This is going to be open season once again, and we need to nominate someone with the experience and the fortitude and the know-how to take whatever they send our way and send it right back."

IRAQ AT ISSUE

Obama, who would be the first black U.S. president, pointed to support from independents and young people and said he would not have to defend a Senate vote authorizing the war in Iraq, like Clinton.

"If John McCain is the (Republican) nominee, then the Democratic Party has to ask itself, do you want a candidate who has similar policies to John McCain on the war in Iraq or someone who can offer a stark contrast?" Obama asked a rally in Wilmington, Delaware.

Even with half the Democratic national convention delegates at stake and more than 40 percent of the Republican, it could be that no candidate clinches the nomination on Tuesday. But a big vote across the board could go a long way toward that goal.

McCain, an Arizona senator, held a 2-to-1 margin in a new national Washington Post-ABC poll. In the Reuters/C-SPAN/Zogby poll, McCain held double-digit leads in New York, New Jersey and Missouri but narrowly trailed former Massachusetts Gov. Mitt Romney in California.

Romney, who was in Missouri, said he would fly west on Monday for one final push in California where he said polls were showing "growing support."

"California is looking good. ... I've got to go after California, why not, they got a lot of delegates," said Romney who cancelled an event in Georgia to make time for a quick dash across the country to California.

McCain invaded Romney's home turf on Sunday night, visiting a pub in Boston as the city halted to watch the New England Patriots in the Super Bowl football championship.

McCain was cautious on whether he would win Massachusetts but felt good about his prospects on Super Tuesday.

"We think we've got a great chance," he said. "We think we may be able to really finish this up Tuesday."

Even as his lead in the polls widened, McCain still faced questions about the depth of his conservatism.

At a Super Bowl party in Maryland Heights, Missouri, Romney returned to his theme of saying it was a fight for the "heart and soul" of the Republican Party and painting McCain as a liberal.

"He's a fine man, just more liberal on a lot of issues than I am," he told the crowd of cheering supporters.

One of the problems facing Romney on Super Tuesday is that he is competing for conservative votes along with former Arkansas Gov. Mike Huckabee. Huckabee said Romney, as a late comer to conservative causes, should recognize him as the true conservative and get out of the race.