Senator John McCain, buoyed by new polls and endorsements, appeared in an increasingly commanding position on Sunday as he headed toward coast-to-coast contests that could effectively hand him the Republican presidential nomination, party officials said. In a display of confidence, Mr. McCain campaigned in the backyard of his chief rival, Mitt Romney.
A sweep of big states by Mr. McCain on Tuesday would reward him with a trove of delegates and could bring the Republican contest to a quick end. That would amount to a remarkable comeback for a campaign that had appeared expired just six months ago.
On the Democratic side, Senators Hillary Rodham Clinton and Barack Obama were enmeshed in a tough national fight, illustrated by polls showing the race had tightened both nationally and in key states voting on Tuesday where Mrs. Clinton had once enjoyed a comfortable lead. They include California, Missouri, New Jersey and Arizona.
Aides to Mrs. Clinton and Mr. Obama said Sunday that they now believed that their contest, unlike the Republicans’, could extend well beyond the multistate contests on Tuesday, the day Mrs. Clinton had once expected to nail down the nomination.
The candidates, already struggling this week to deal with the challenges of campaigning in more than 20 states that hold contests on Tuesday, truncated their schedules in deference to the Super Bowl game between the New England Patriots and the New York Giants, with home audiences in some of the biggest states that vote Tuesday: Massachusetts, New York, Connecticut and New Jersey.
But they certainly made the most of their shortened time, holding rallies, starting new television advertisements and, in Mr. Obama’s case, accepting another endorsement from the Kennedy family. Maria Shriver, the wife of Gov. Arnold Schwarzenegger of California and daughter of Eunice Kennedy, announced her support, adding force to Mr. Obama’s growing strength in that state.
Ms. Shriver appeared at a rally in Los Angeles with Michelle Obama, Oprah Winfrey and Caroline Kennedy that reflected another attempt by Mr. Obama to undercut Mrs. Clinton’s advantage among female voters.
Mr. Romney, in a last-minute switch of plans, decided to fly to California on Monday. His aides argued that Mr. McCain had left that flank vulnerable by deciding to head to Massachusetts. And Mr. Romney tried to discredit Mr. McCain among conservatives by attacking his record on immigration and the environment.
"Yesterday, Barack Obama said there’s not a dime’s worth of difference between he and Senator McCain on illegal immigration," Mr. Romney said at a rally outside Chicago. "I’m afraid it’s going to be real hard to win the White House if there’s not much difference between our nominee and theirs, and that’s why I’m going to make sure that we stand for Republican ideals and win the White House on that platform."
Mr. McCain was already looking to the future, incorporating an unusual attack on Mrs. Clinton as part of his standard denunciation of federal budget earmarks. He made the remarks in Fairfield, Conn., where he was escorted by Senator Joseph I. Lieberman, a Democrat-turned-independent.
"In her short time in the United States Senate, the senator from New York, Senator Clinton, has gotten $500 million worth of pork-barrel projects," Mr. McCain said. "My friends, that kind of thing is going to stop when I’m president of the United States."
Mrs. Clinton and Mr. Obama both tussled, at a distance, over which of them would be the better general election candidate in a campaign that many Democrats now think will be waged against Mr. McCain, a Republican with a history of drawing strong support from independent voters. Mrs. Clinton, her voice hoarse and raspy, responded to a question about this by pointing to her unexpected victory in her first Senate race.
"You know, I hear all these folks talking about who is and isn’t electable — and they said all the same things in New York," she added, noting that voter support for her grew and deepened over time. "I trust the voters — frankly, that’s who matters."
"One thing about me, I’ve already been through tough campaigns, and I think that says something about me," she said. "My opponent hasn’t had to go through that baptism of fire. And in a general election, you know what’s going to happen to our nominee. Let’s not kid ourselves."
Mr. Obama raised similar questions about Mrs. Clinton as he, too, raised the specter of a McCain candidacy this fall and reminded his audience that Mrs. Clinton had initially voted to support the war in Iraq.
"If John McCain is the 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 somebody who can offer a stark contrast?" Mr. Obama said. "See, when I’m the nominee, John McCain won’t be able to say that "You were for this war in Iraq,’ because I wasn’t. He won’t be able to say I followed the Bush-Cheney doctrine of not talking to leaders we don’t like, because I don’t."
A blizzard of state and national polls showed Mrs. Clinton and Mr. Obama in a tight race, but with significant voters still undecided about what they were going to do. In the CBS News poll, Mrs. Clinton and Mr. Obama are each the choice of 41 percent of the Democratic primary voters. Mr. Obama has narrowed the gap in a number of key states voting Tuesday.
The polls this weekend suggest the Republican contest is less in flux, with Mr. McCain having staked out a significant lead over Mr. Romney and a third candidate, former Gov. Mike Huckabee of Arkansas.
Mr. McCain was endorsed Sunday by Pete Wilson, former governor of California, Senator Norm Coleman of Minnesota and the editorial page of Newsday.
"It’s not over yet, but it is certainly trending that way," said Todd Harris, who was a senior adviser to Fred D. Thompson of Tennessee, who dropped out of the race on Jan. 22. His view was echoed throughout the day by Republican leaders outside and inside Mr. McCain’s camp.
Mr. Romney’s situation was complicated by Mr. Huckabee, who — apparently seeing an opening should Mr. Romney falter
— vowed again on Sunday to stay in the race. Mr. Huckabee, a Baptist minister, has drawn conservative support from Mr. Romney and challenged his conservative credentials.
"I want to say something to Mitt Romney: The people who support me are supporting me because they know where I stand on human life and they don’t know where you stand," he told reporters in Georgia.
Mr. McCain made a pre-Super Bowl stop at the Green Dragon, a Boston pub. The visit was a display of confidence by the senator from Arizona, given the spare number of days left before the vote. It reflected the calculation of Mr. McCain’s advisers that a loss by Mr. Romney in his home state on Tuesday could, if combined with other losses in other big states, effectively force him from the race.
Mr. Romney dismissed the visit as a stunt as his aides warned that Mr. McCain would come to regret flying to Massachusetts rather than spending time in California.
"I don’t know why he’s campaigning in Massachusetts — there are 22 states voting," Mr. Romney said. "I expect that I’m to win in Massachusetts. I’m going to where I think the most delegates are up for grabs. To me this is not about trying to tweak somebody or get in their head. This is about getting delegates and becoming the nominee."
Mr. Romney and his aides planned an aggressive final 24 hours, including using automatic telephone calls by prominent supporters to raise questions about Mr. McCain’s conservative credentials and his temperament — including one taped by Rick Santorum, the former senator from Pennsylvania.
"John McCain voted against the president’s tax cuts, worked with Ted Kennedy to pass what many people call amnesty for illegal aliens," Mr. Santorum says in the call, adding, "As a conservative, I don’t agree with McCain on many issues, and I don’t think he has the temperament and leadership ability to move the country in the right direction."
On the Democratic side, Mr. Obama, flexing his financial muscle, bought Super Bowl advertisements in 24 markets to present an advertisement highlighting his opposition to the war and pledging to fight global warming. "We want to turn the page," Mr. Obama says in the spot, speaking under a musical track that is faster-paced than most political advertisements. "The world as it is is not the world as it has to be."
At Mrs. Clinton’s campaign event in Missouri, a woman used a profanity in describing Mr. Bush as she accused him of signing a secret agreement to merge the United States, Mexico and Canada into a new country. Mrs. Clinton did not respond to the cursing; she appeared to grin slightly at first as the crowd cheered, and her face went blank before assuring the questioner that her fears about the merger were ill-placed.
"Let me say I’ve heard that story and there’s not a lot of truth to it," she said. "If I am president, if I discover there is such an agreement, it’ll be gone in a bird-dog minute."
The Republican National Committee quickly sent around a news report noting that she said nothing when the woman swore about the president.