Marco Rubio amassed increasing support for his candidacy while Ted Cruz's campaign showed signs of struggle as Republican presidential candidates crisscrossed Nevada on Monday on the final day of campaigning ahead of the state's GOP caucuses.

Cruz told reporters Monday he has asked his campaign spokesman, Rick Tyler, to resign for tweeting a story that falsely alleged Rubio insulted the Bible.

"We are not a campaign that is going to question the faith of another candidate for president," he said, calling Tyler "a good man" and noting that he deleted the tweet once he discovered it was false.

Tyler did not return telephone, text or email messages left by The Associated Press seeking comment.

It's not the first time that Cruz's campaign has been accused by rivals of using questionable tactics. Cruz apologized to GOP hopeful Ben Carson earlier this month after his campaign promoted a news story suggesting that Carson was getting out of the race. Cruz's campaign has also acknowledged creating a website that used a computer program to create a fake picture of Rubio shaking hands with President Barack Obama.

Speaking during a campaign stop in Elko, Nevada, earlier Monday, Rubio criticized Cruz for the incident and asked whether Cruz would fire anyone involved.

"It's every single day something comes out of the Cruz campaign that's deceptive and untrue, and in this case goes after my faith," Rubio told reporters when asked about the incident. "I guess one of their spokespersons apologized, and I accept their apology."

Republican front-runner Donald Trump lashed out at Cruz over Twitter on Monday, saying that Cruz "has now apologized to Marco Rubio and Ben Carson for fraud and dirty tricks. No wonder he has lost Evangelical support!" He reiterated his calls for disqualifying Cruz because of "his fraudulent win in Iowa."

Trump's campaign manager Corey Lewandowski said Monday that the billionaire businessman has been seeking advice from former New York City Mayor Rudy Giuliani as he gradually expands his tight inner circle.

Meanwhile, establishment heavyweights continued to back Rubio, with many saying they see him as the candidate who can unite a disharmonious Republican Party. Arkansas Gov. Asa Hutchinson and Utah Sen. Orrin Hatch were the latest to endorse Rubio. South Florida's three Cuban-American members of Congress also said that they shifted their support to Rubio, having previously supported for former Florida Gov. Jeb Bush's now defunct bid.

Former Sen. Bob Dole told ABC News on Monday that he too had been backing Bush, but he's now supporting Rubio because "he wants to grow the party as opposed to Cruz. I don't know what he wants to grow."

Since Friday, Rubio has added 12 new Congressional or gubernatorial endorsements, while Trump and Cruz have added none.

While five men officially remain in the race for the Republican nomination, House Majority Leader Kevin McCarthy billed it as a two-man contest between Trump and Rubio. Speaking to MSNBC's "Morning Joe" Monday, McCarthy said Trump's victory and Rubio's second-place finish in Saturday's South Carolina primary dealt a blow to Cruz's strategy to win the nomination. The California congressman predicted voters in Florida, Rubio's home state, would determine whether Rubio continues or Trump easily rolls on to the nomination.

Cruz on Saturday characterized a two-man contest as well – between him and Trump. But Rubio, who campaigned in South Carolina alongside the state's GOP Gov. Nikki Haley, has continued to capitalize on the momentum from his second-place finish.

As the Associated Press reported:

With Jeb Bush gone from the race, Rubio was hoping to pick off past donors to the Bush campaign and looking to benefit as well from a cessation in the millions of dollars in negative ads run by the Bush campaign and its allies. Rubio also suggested it was only a matter of time before Ohio Gov. John Kasich and retired neurosurgeon Ben Carson folded as well. He hinted it would be better to get that winnowing over with, saying, "the sooner we can coalesce, the better we're going to be as a party in general." "It's going to happen one way or another," he said. Not so fast, Governor Kasich countered. "We're getting big crowds everywhere we go," he insisted, listing Vermont, Massachusetts, and Virginia as places he can shine.

Across the country, Ohio Gov. John Kasich turned his attention to Virginia, which is among the dozen states voting on March 1.

Kasich, who finished second in New Hampshire's Republican primary, won the endorsement of Tom Ridge, a former Homeland Security secretary and Pennsylvania governor.

Ridge had supported Bush for president since early 2015 and campaigned with him in South Carolina. Bush quit the race Saturday after a disappointing finish in the Iowa, New Hampshire and South Carolina primaries.

Kasich's campaign says Ridge is signing on as a national co-chairman.

Democratic presidential hopefuls coming off a tight battle for Nevada kicked off the week on opposite ends of the country Monday. Hillary Clinton was fundraising in northern California, while Bernie Sanders held a rally in Massachusetts, another Super Tuesday state. South Carolina votes in the next Democratic primary on Saturday.

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Riccardi reported from Las Vegas. Associated Press writers Vivian Salama in Washington, Scott Bauer in Madison, Wisconsin; David Eggert in Michigan, Andrew Demillo in Little Rock, Arkansas; and Alanna Durkin, Michael Blood and Matthew Barakat in Virginia contributed to this report.