"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.
