Donald Trump’s rivals desperately are trying to sideline each other in a last-ditch bid before Super Tuesday to bolster their own numbers and mount a viable challenge against the billionaire front-runner -- each calling on the other to get out of the race, now.

Ahead of a must-shine debate Thursday night, Marco Rubio, Ted Cruz and John Kasich all claimed to be the only candidate who can derail Trump, whose momentum has intensified with three straight wins in the four previous Republican contests.

Only Ben Carson has held back from the fray -- instead characterizing his distant finishes as evidence that "things are starting to happen."

Ohio Gov. Kasich’s campaign, despite performing poorly in the South Carolina and Nevada contests, even issued a defiant call Thursday for Rubio to drop out

.@JohnKasich is the ONLY candidate who can beat @realDonaldTrump on 3/15. @marcorubio needs to do the right thing & suspend campaign,” Kasich spokesman Chris Schrimpf tweeted, referring to the March 15 primary in Florida.

 

The state is considered a must win for the Florida senator, but a new Quinnipiac University poll showed Trump leading Rubio 44-28 percent among primary voters there.

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Kasich adviser John Weaver also tweeted that “with the Florida implosion, Marco Rubio needs to withdraw from the race” and support Kasich.

That’s not likely to happen.

Rubio, despite not having won a primary contest yet, has been steadily racking up endorsements from fellow lawmakers and big-name Republicans, trying to position himself as the best candidate to win a general election in November and the best choice to take on Trump.

Rubio, who has finished second in each of the last two nominating contests, acknowledged during a Fox News forum with host Megyn Kelly on Wednesday that Trump is "the frontrunner and I'm the underdog, but I've been an underdog my entire life."

Rubio added that his campaign "would not allow the conservative movement to be defined by a nominee who isn't a conservative."

The candidates square off Thursday night in Houston, for a CNN-Telemundo debate.

For some, it could represent their last debate chance to define themselves as the anti-Trump candidate, as the field charges next into the more than a dozen states holding contests on Super Tuesday. Anything close to a sweep by Trump on Tuesday would be devastating for the other remaining candidates.

Texas, Georgia and other delegate-heavy states are on the line that day.

At the Fox News forum on Wednesday, Texas Sen. Cruz argued that he – not anybody else – is the best alternative candidate since he’s the only other one who has won a primary contest, the Iowa caucuses.

He said his campaign was "the only campaign that can beat Donald [and] has beat Donald."

Trump, though, all along has argued against the conventional wisdom that if other candidates drop out, their support will gravitate to the other non-Trump candidates. Trump says he would pick up some of that support as well.

Larry Sabato, director of the University of Virginia Center for Politics, sided with Trump in that regard.

“Trump will get a share. He might not get a majority, but he’ll get a share,” he told Fox News.

The pressure is strongest, though, on candidates Kasich and especially Carson, whose campaign has steadily fallen in the polls and has performed poorly in the first four contests.

Carson, though, has appeared immune to the political pressure or the poor showings in South Carolina and Nevada. After placing fourth in Nevada, Carson nevertheless declared to supporters that “things are starting to happen.”

Carson seemed to level a bit more with supporters in a fundraising email on Wednesday, saying without a strong showing on Super Tuesday, “it's going to be very tough for my campaign. That's the honest truth."

Kasich, though, told Fox News that “the people calling for me to get out are the people who are inside the Beltway ... I'm certainly not listening to a bunch of lobbyist insiders."