WASHINGTON (CNN) -- Consistently defiant rivals are doing little to hamper Mitt Romney's momentum ahead of nominating contests on Tuesday in what is shaping up to be more of a general election fight between the former Massachusetts governor and President Barack Obama's re-election team.
Rick Santorum and Newt Gingrich on Sunday insisted they will stay in the race until a candidate reaches 1,144 delegates -- the necessary threshold to win the Republican presidential nomination -- while acknowledging Romney currently has the advantage.
Santorum cited the Democratic nomination fight in 2008 and Mike Huckabee's Republican presidential bid in 2008 to push back against the narrative the former Pennsylvania senator is hurting the party.
"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," Santorum said on "Fox News Sunday."
"And guess what? 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, who scaled back his campaign infrastructure this past week and has tempered his attacks on Romney, on Sunday said he is launching a come-from-behind strategy in the race, comparing his long-shot bid to Kansas' NCAA basketball Final Four victory on Saturday night.
But Romney's competition faces a steep delegate climb. According to the latest CNN estimate, Romney currently has 571 delegates in his column, compared to Santorum's 264, Gingrich's 137 and Ron Paul's 71. To clinch the nomination, Santorum needs 880 delegates, or 72% of the remaining advocates at stake. Gingrich would need 1,007 delegates, or 83% of the remaining delegates.
Voters in Maryland, Wisconsin and Washington, D.C., will cast ballots on Tuesday and Romney is expected to win all three. There is then a three-week gap before the next set of elections.
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 specifically cited Texas, Arkansas, West Virginia, North Carolina, Indiana, Kentucky and his home state of Pennsylvania as regions in which he could show well.
However, if Romney reaches the desired delegate threshold, Santorum conceded he would end his campaign, as did Gingrich.
"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."
The much-watched contest in Wisconsin, where Romney has a lead in statewide polling, will potentially place an additional win in Romney's column, further solidifying his position as the GOP frontrunner.
The most recent poll from NBC News/Marist out of Wisconsin, where 42 delegates are at stake, indicated Romney edging out his opponents with 40% support from likely primary voters, followed by Santorum's 33%, Paul's 11% and Gingrich's 8%.
Romney himself predicted a Wisconsin win aided by 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 elections follow a week of high-profile endorsements for Romney. He received the backing of Florida Sen. Marco Rubio, former President George H.W. Bush and Wisconsin Rep. Paul Ryan, chairman of the House Budget Committee.
Their message: It is time to get behind one candidate to avoid harm to the party, also a consistent narrative on Sunday.
Ryan predicted an essential end to the primary season 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, R-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 told CNN Chief Political Correspondent Candy Crowley on "State of the Union."
Sen. Ron Johnson of Wisconsin added his name to the growing list of Romney supporters on Sunday and pivoted 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 this past 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 surrounding 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' "Face the Nation."
Romney continued to hammer the current White House occupant specifically over health care and the economy throughout the weekend, a chorus echoed by Santorum and Gingrich during the Wisconsin Faith & Freedom Coalition on Saturday.
"All in all, President Obama prolonged the recession and slowed the recovery," Romney said Friday while campaigning in Wisconsin. "These troubling facts are President Obama's legacy and now our shared history, and as much as we'd like to, we can't undo what's happened these past three years."
CNN's Gregory Wallace, Ashley Killough and Rachel Streitfeld contributed to this report.