Senator McCain barnstormed through a skeptical South yesterday, campaigning for a Super Tuesday knockout in the Republican presidential race. Senators Obama and Clinton worked the West on the final weekend before primaries and caucuses in more than 20 states.

“I assume that I will get the nomination of the party,” Mr. McCain told reporters, the front-runner so confident that he decided to challenge rival Mitt Romney in his home state of Massachusetts.

Mr. Romney, on the other hand, celebrated a caucus victory in Maine and told reporters he plans to do well Tuesday, “planning on getting the kind of delegates and support that shows that my effort is succeeding, and taking that across the nation. … I am encouraged by the support which I’m seeing grow for me.”

Mrs. Clinton stressed pocketbook issues, the home mortgage crisis in a discussion with voters in a working class neighborhood, and health care at a noisy rally in California attended by a former Los Angeles Lakers basketball star, Earvin “Magic” Johnson. “This is a cause that is the central passion of my public life,” she said, and jabbed at Mr. Obama on the issue.

“My opponent will not commit to universal health care. I do not believe we should nominate any Democrat who will not stand here proudly today and commit to universal health care,” she said in the continuation of a monthslong debate over which candidate’s plan would result in wider coverage among the millions who now lack it.

Mr. Obama stopped in Idaho, where caucuses offer a mere 18 delegates on Tuesday, and he worked to reassure Westerners on two fronts.

“I’ve been going to the same church for more than 20 years, praising Jesus,” he told an audience in Boise, warning his listeners not to believe e-mails that falsely say he is a Muslim.

In a region of the country where hunting is a way of life, he also said he has “no intention of taking away folks’ guns.” The Illinois senator did not mention his support for gun control legislation.

The two remaining Democratic rivals compete in primaries in 15 states as well as caucuses in seven more plus American Samoa on Tuesday, the busiest day of this or any other nominating campaign. A total of 1,681 delegates is at stake, including 370 in California alone, and the two campaigns have said they do not expect either side to emerge with a lock on the nomination.

Both have already begun turning their attention to February 12 primaries in Virginia, Maryland, and the District of Columbia.

Mr. Obama told reporters on a flight from Boise to Minneapolis that he thinks the race for votes on Tuesday is getting tighter, even though the schedule seems to favor the more well-known Mrs. Clinton. “I don’t think that there is any doubt that we’ve made some progress. I don’t think that there’s any doubt that Senator Clinton — she’s still the favorite,” he said on the way to a rally that drew 20,000 people to the Target Center.

The Republican political landscape is different for Messrs. McCain, Romney, a former Arkansas governor, Michael Huckabee, and Rep. Ron Paul of Texas, with nine of the 21 contests on the ballot awarding delegates winner-take-all to the top vote-getter.

At a stop in Minnesota, Mr. Romney called his caucus victory Saturday in Maine, where he took little over 50% of a presidential preference vote, “a people’s victory,” noting that it came despite McCain’s endorsement by the state’s two American senators.

“It is, in my view, also an indication that conservative change is something that the American people want to see. I think you’re going to see a growing movement across this country to get behind my candidacy and to propel this candidacy forward,” Mr. Romney said. “I think it’s a harbinger of what you’re going to see on Tuesday.”

Without mentioning him by name, Mr. Romney also took a jab at Mr. McCain, telling an audience in Edina: “I don’t think we win the White House by getting as close to Hillary Clinton as we can be without being Hillary Clinton.”

Mrs. Clinton and Messrs. Obama, Huckabee, and Paul participated via satellite in a televised youth forum during the evening. The event was sponsored by MTV, The Associated Press and MySpace.

Each appearing separately, the Democrats pitched their college aid proposals; Mr. Huckabee, his theory of “vertical” leadership that breaks through the “horizontal” politics of left and right; and Mr. Paul, his belief that government is best when it gets out of people’s way.

Mrs. Clinton, noting Democrats are choosing between a female and a black candidate, said: “Whichever of us gets the nomination, we are making history,” and asserted she is the best equipped to lead. Equally mindful of history, Mr. Obama said the contest is not about the race or the sex of the candidates.

If it were just about his race, he said, “I wouldn’t have to answer questions. I could just show up.”

Mr. McCain’s rivals have essentially conceded him New York, New Jersey, Connecticut, Delaware, and Arizona, five winner-take-all states with 251 delegates combined.

That left Mr. McCain free to spend yesterday in Mr. Huckabee’s probable area of strength, Tennessee, Alabama, and Georgia. All three are home to large numbers of evangelical voters who have been slow to swing behind the Arizona senator on his march through the early primaries and caucuses.

He worked to reassure conservatives, telling them he had a 24-year record in the Senate of “fighting for the rights of the unborn” and boasting he never asked for a single earmark or pork barrel project for his home state of Arizona.

As for the slowing economy, he said the Senate must “stop fooling around and pass the president’s stimulus package …. and restore some confidence.”

Mr. McCain made no mention of Mr. Romney, the former Massachusetts governor who is his closest pursuer in the race, or of Mr. Huckabee, the Baptist preacher-turned-politician.

In Tennessee, Mr. McCain made a pitch for the supporters of a campaign dropout, Fred Thompson, a former Tennessee senator. “He is a fine man. I had the distinct pleasure and honor of sitting next, my desk right next to Fred Thompson for eight years in the United States Senate,” he said. Mr. Thompson has not endorsed any of the remaining candidates.

Before campaigning in Minnesota, Mr. Romney attended the funeral of the Mormon Church president, Gordon B. Hinckley, in Salt Lake City. Mr. Romney would be the first Mormon to sit in the White House if he wins the presidency.

Mr. Huckabee campaigned across Alabama, taking thinly veiled swipes at Messrs. McCain and Romney.

“You really would like to get a president to agree with himself on some issues,” he said in a reference to Mr. Romney, who has switched positions on key issues since he ran against Senator Kennedy in Massachusetts in 1994. As for Mr. McCain and the need to control federal spending, he said, “It doesn’t make sense that someone would be sent to the White House who has a Washington address.”

Mr. McCain emerged as the front-runner in the Republican race with a victory in the winner-take-all primary in Florida last Tuesday. In the days since, he has begun collecting endorsements from establishment figures ranging from Governor Schwarzenegger of California to a former senator, Don Nickles of Oklahoma.

But a significant number of conservatives remain vocally opposed to him, and Mr. Romney hopes to take advantage of their unwillingness to swing behind a longtime party maverick.

“It’s going to destroy the Republican Party,” a radio show host, Rush Limbaugh, has said of a McCain nomination. A conservative author and commentator, Ann Coulter, has said she would prefer Mrs. Clinton in the White House over Mr. McCain, adding, “I will campaign for her.”