WASHINGTON (CNN) -- Mitt Romney and his main challenger for the Republican presidential nomination focused their campaign efforts Monday on Wisconsin, the main prize among three contests Tuesday before a three-week break in the primaries.
Romney and Rick Santorum held events across Wisconsin on the eve of the winner-take-all primary offering 42 delegates. Maryland and the District of Columbia also will hold primaries Tuesday, and Romney is leading in all three contests, according to recent polling.
Despite Romney's big lead in delegates and growing list of endorsements from prominent Republicans, both Santorum and fellow conservative challenger Newt Gingrich insist they will remain in the race until the former Massachusetts governor wins the 1,144 delegates necessary to secure the nomination.
In an interview Monday on Fox News, Santorum guaranteed he would win the primary in his home state of Pennsylvania later this month.
On Sunday, Santorum cited the Democratic nomination fight in 2008 and Mike Huckabee's Republican presidential bid in 2008 to push back against the narrative that he is hurting the party by staying in the race.
"Four years ago, everyone said, 'Oh, we got to wrap this thing up' and we did, and John McCain was the nominee. And Barack Obama and Hillary Clinton went in the summer and pounded it out," the former U.S. senator from Pennsylvania said on "Fox News Sunday."
"And guess what?" Santorum continued. "They came up with the best candidate and we came up with someone who, well, just wasn't able to win. We don't need to repeat that again."
Gingrich, the former House speaker, who scaled back his campaign infrastructure last week, said Sunday he is launching a come-from-behind strategy.
He compared his long-shot bid to the University of Kansas men's basketball team, which squeaked past Ohio State on Saturday night to set up Monday's NCAA tournament championship showdown with top-seeded Kentucky. However, Romney's competitors face a steep delegate climb.
According to the latest CNN estimate, Romney has 571 delegates in his column, compared with Santorum's 264, Gingrich's 137 and 71 for U.S. Rep. Ron Paul of Texas.
To clinch the nomination, Santorum would need 880 delegates, or 72% of the remaining delegates at stake. Gingrich would need 1,007 delegates, or 83% of the remaining delegates.
Further complicating the bids by the trailing candidates is a three-week break in primary voting after Tuesday's contests in Maryland, Wisconsin and the district. The next showdowns are on April 24 in Pennsylvania, New York, Connecticut, Delaware and Rhode Island.
Whatever the outcome Tuesday, Santorum said, he will not end his White House bid and instead will channel his energy into upcoming states that he said look "very, very good for us."
He cited Texas, Arkansas, West Virginia, North Carolina, Indiana, Kentucky and Pennsylvania as states where he could show well.
However, if Romney reaches the necessary delegate threshold, both Santorum and Gingrich conceded they would end their campaigns.
"If Gov. Romney gets that required number, then without a doubt, if he's at that number, we'll step aside," Santorum said on NBC's "Meet the Press." "Right now, he's not there. He's not even close to it."
Wisconsin represents the latest showdown in the contest, with Santorum needing a victory to regain momentum against the growing Romney lead. A Romney triumph there Tuesday, which is expected, would further solidify his front-running status heading into the upcoming lull in primaries and increase pressure on Santorum and Gingrich to drop out.
The most recent poll from Marist out of Wisconsin showed Romney with 40% support from likely primary voters, followed by Santorum's 33%, Paul's 11% and Gingrich's 8%.
A new Santorum ad airing statewide in Wisconsin links Romney to Obama, with a narrator describing an agenda of mandated health care, cap-and-trade energy policy and tax increases while standing against a large picture of Obama. The picture then changes to one of Romney.
Responding to the ad, Romney campaign spokeswoman Andrea Saul slammed Santorum as reaching a "point of desperation."
"It is sad to see him completely lose his bearings and revert to patently false claims," Saul said.
Romney, meanwhile, targeted Obama's health care reform law in an interview with radio WISN, saying he hoped the Supreme Court "puts a stake through its heart so we can get rid of this thing once and for all."
The high court held three days of hearings last week on the 2010 health care law, which many conservatives despise because it requires people to buy health insurance or pay a fine.
Romney has called for the law's repeal, but Santorum has repeatedly criticized Romney for a Massachusetts health care law, passed while he was governor, that the White House says was the model for the federal law.
Over the weekend, Romney predicted a Wisconsin win based on support "growing stronger and stronger."
"This was an uphill battle for me if you look back three or four weeks ago, and now we're looking like we're going to win this thing on Tuesday," Romney said in Fitchburg, Wisconsin, on Saturday. "I got a good boost from the folks in Illinois, and if I can get that boost also from Wisconsin, I think we'll be on a path that will get me the nomination well before the convention."
Tuesday's votes follow a week of high-profile endorsements for Romney. He received the backing of U.S. Sen. Marco Rubio of Florida, former President George H.W. Bush, and Rep. Paul Ryan of Wisconsin, chairman of the House Budget Committee. Their message: It is time to get behind one candidate to avoid harm to the party. Ryan predicted the GOP campaign will effectively end if Romney sweeps Tuesday's contests.
"If he gets a big delegate count, which I think he'll get, then we believe, as conservatives, that we should coalesce around our nominee and focus on the task at hand, which is the fall election, and not drag this thing out, which I think becomes counterproductive," Ryan said on CNN's "State of the Union."
Although he did not offer his official endorsement, Senate Minority Leader Mitch McConnell of Kentucky said Romney will be an "outstanding nominee."
"It's absolutely apparent that it's in the best interests of our party at this particular point to get behind the person who is obviously going to be our nominee and to begin to make the case against the president of the United States," McConnell said on "State of the Union."
Sen. Ron Johnson of Wisconsin added his name to the growing list of Romney supporters Sunday and turned the conversation toward the general election.
"I'm looking forward to making sure that President Obama is a one-term president," Johnson said on NBC's "Meet the Press."
The Obama administration took aim at Romney last week through surrogates, most notably Vice President Joe Biden, who accused Romney of being "consistently wrong" throughout his career as an investor, businessman, governor and presidential candidate.
On Sunday, Biden labeled Romney "a little out of touch," pointing to his proposed policies on health care, the economy and the auto bailout.
"I can't remember a presidential candidate in the recent past who seems not to understand, by what he says, what ordinary people are thinking about and are concerned about," Biden said on CBS's "Face the Nation."
Romney continued to hammer the current White House occupant over health care and the economy, a theme echoed by Santorum and Gingrich to the Wisconsin Faith & Freedom Coalition on Saturday.