Americans vote on Tuesday in what is deemed the most pivotal day in the presidential nominating process, with front-runners Hillary Clinton and Donald Trump hoping to finish off their challengers.
Speaking with host Jake Tapper, Cruz reminded voters he's the only candidate who can and has beaten Donald Trump, but added he "will support the Republican nominee". Bernie Sanders of Vermont. A big chunk of the electorate votes early in Arkansas, Georgia, Tennessee and Texas, and those early votes will show up quickly in the count, increasing the chances for fast calls in those states. Republicans vote in Alaska and Democrats in Colorado.
How completely will Trump dominate?
Mr Trump hit back hard against Mr Rubio, calling him "Little Marco", mocking him for sweating on the campaign trail and warning Mr Rubio could not stand up to strong men like Russian President Vladimir Putin.
The Florida senator warned supporters in Tennessee that U.S. media and critics will jump on Mr Trump "like the hounds of hell" if he wins the nomination.
Clinton, however, appeared to be looking on toward the general election - and Donald Trump.
If Trump's rebel yell attracts enough support to make him the victor or even a close second, Cruz will face sharp questions about his viability.
The attack echoes themes that Rubio, who is trying to unite the GOP's anti-Trump forces under his own banner, has made as he has addressed swelling crowds in suburban areas.
In the latest controversy, Mr Trump came under withering criticism from Republican and Democratic candidates alike for not immediately disavowing the support of Mr David Duke, a white supremacist who once led the Ku Klux Klan.
Given the sheer number of ballots going on, this day can give us the first concrete indication of which candidates are most likely to be contesting the presidential election later this year.
After days of relentlessly building expectations, the pressure is now on the Texas senator to perform.
This year, Arkansas and Minnesota are the remoras on the shark that is Texas, the hangers-on as the attention goes to the big state.
"I am not going to vote for Trump under any circumstances", Wehner went on. He wouldn't win many delegates, still, but he'd be able to make the argument that he should continue in the race. He is leading in polling in at least eight of the 11 Super Tuesday states.
Trump leads in all opinion polls except in Cruz's home state of Texas.
John Cook, a retired investment adviser in Houston, says he chose Rubio because Jeb Bush has dropped out of the race.
And the Texas prize for Trump is found as much in its symbolism as in its delegate distribution.
The Republicans also hold semi-closed primaries in Georgia and MA, while the primary in Oklahoma is closed, meaning only registered Republicans can vote there. He campaigned with Governor Nikki Haley through the state hoping to bring people to his side.
But a nightmare scenario for the Florida senator and his backers is an 0-for-11 night, coupled with Cruz notching more than one victory.
In the entire sample, 32 percent described themselves as Democrats; 29 percent described themselves as Republicans; and 39 percent described themselves as independents or members of another party.
Rubio won't be leaving the race anytime soon, no matter what happens Tuesday.
If Rubio can secure some delegates, he could very well start to catch up to Trump in the delegate race on March 15, when several large states hold their primaries.
But his path is increasingly narrow.
Can Clinton put it on ice?
"The case against Trump will be prosecuted on two levels", said Geoff Garin, a Democratic pollster and Mrs. Clinton's chief strategist in 2008. Less clear is whether her winning streak will dampen Sanders' previous advantage among the five other states at stake - Colorado, Minnesota, Oklahoma, Massachusetts and Sanders' home state of Vermont. But only Vermont is a guaranteed win for him.
Alabama is another Southern state with a high African-American population where Clinton is expected to cruise to victory over Sanders. He can lose by 14 points in the vote and only be down 10 in the Texas at-large delegate count.
Barring disaster, Clinton's delegate lead is certain to grow Tuesday.
Sidney, the Sanders super fan, paused for a few seconds when asked if she could envision herself voting for Clinton.
Check out the election-night speeches by Clinton and Trump.
The Sanders campaign announced Monday morning it had raised $36 million in February and set a goal of $40 million. The problem is he needs to find new voters.