Tuesday is the biggest day so far in the 2016 U.S. presidential nominating campaign, with challengers looking to curb the momentum for the front-running candidates, billionaire real estate mogul Donald Trump in the Republican race and former Secretary of State Hillary Clinton in the Democratic contest.
Eleven states vote on what is called "Super Tuesday," awarding nearly a quarter of the delegates the presidential contenders need at the Republican and Democratic national party conventions in July to win their party nominations before the victors square off in November's national election.
"It's the most important day in the campaign," a conservative firebrand, Texas Senator Ted Cruz, told CNN on Sunday.
Cruz said he expects to beat Trump in the Republican primary in his home state, the large southwestern state where more convention delegates are at stake in both parties' primaries than in any of the other 10 states voting Tuesday.
Republican presidential candidate Ted Cruz eyes a supporter reaching to shake his hand during a rally at Liberty Plaza in Atlanta, Feb. 27, 2016.
With the exception of Texas, political surveys show Trump leading throughout the states that vote Tuesday, but Cruz said he is "running neck and neck with Donald in a number of states."
He disparaged Trump, a one-time television reality show host, as likely to lose to Clinton in the November election and said Trump "doesn't even know what he'd do" as president should he win the race to the White House.
But Cruz acknowledged in an interview with CBS' Face the Nation that "there is no doubt that if Donald steamrolls through Super Tuesday, wins everywhere with big margins, that he may well be unstoppable" to win the Republican nomination.
Trump has already won three of the first four states to hold Republican nominating contests.
Florida Senator Marco Rubio, who is calling Trump a "con artist," is not predicting he will win any of the Super Tuesday contests, but told CBS that Trump, with a penchant for insulting his opponents, would not eventually be the Republican nominee.
FILE - Republican presidential candidate, Sen. Marco Rubio, R-Fla., is welcomed at a rally in Houston, Texas, Feb. 24, 2016.
"It's not going to happen," Rubio said. He added that if Trump were to become the Republican presidential nominee, "it will split us [the Republican Party] and splinter us in a way that we may never be able to recover and the Democrats will be joyful about it."
Rubio is making campaign stops in several states before the Tuesday elections, hoping to win convention delegates that are being awarded proportionally based on the vote count in each state.
He is also looking ahead to March 15, when his home state of Florida votes in a winner-take-all primary, although surveys taken before Rubio unleashed a verbal assault on Trump at last week's debate show Trump ahead in the southeastern U.S. state as well.
The Republican front-runner drew questions on Sunday news shows about support for his campaign voiced by white supremacist David Duke and Trump's passing along a quote from Benito Mussolini, the World War Two-era Italian dictator, on Twitter.
Trump, who has never held public office, declined a chance to disavow Duke's embrace, saying he did not know him and knows "nothing about white supremacists."
Trump said he retweeted the quote from the fascist Mussolini, "It is better to live one day as a lion than one hundred years as a sheep,, because he thought it was a "very interesting" remark
Clinton scored a resounding victory in Saturday's Democratic primary in the Atlantic coastal state of South Carolina, beating her lone rival Vermont Senator Bernie Sanders, a self-declared democratic socialist, by a 3-to-1 margin.
She is favored in most of the 11 states voting in Tuesday's Democratic contests, but Sanders's campaign said it has a good shot at winning five of the states, especially ones outside of the southern tier of states where black voters favoring Clinton comprise a large part of the Democratic electorate.
Democratic U.S. presidential candidate Bernie Sanders addresses supporters during a campaign rally in Rochester, Minn., Feb. 27, 2016.
Sanders is expected to win in Vermont, his small home state in the Northeastern U.S. where few delegates are at stake, and also contend in the neighboring state of Massachusetts, the Western state of Colorado and the northern state of Minnesota.
Clinton, in celebrating her South Carolina victory, seemed to be looking ahead to a general election match-up with Trump, who also is predicting a face-off with her.
Clinton, the country's top diplomat from 2009 to 2013, mocked Trump's campaign slogan, "Make America Great Again" and his call to build an impenetrable wall along the southern U.S. border with Mexico to halt the stream of illegal immigrants into the United States.
"Despite what you hear, we don't need to make America great again. America has never stopped being great," she said. "But we do need to make American whole again. Instead of building walls, we need to be tearing down barriers."
WATCH: Hillary Clinton overwhelms Bernie Sanders in South Carolina primary