In the midst of a weeklong march through Florida to save his candidacy, Florida Sen. Marco Rubio concedes that he went too far in insulting Donald Trump in the last two weeks of campaigning.

"My kids were embarrassed by it and if I had it to do over again I wouldn't," Rubio said on MSNBC Wednesday night.

After a spirited debate on Feb. 25 in Houston in which Rubio mostly stuck to substantive attacks on the front-runner, the following day he called Trump a "con artist" and proceeded over the next several days to get more personal in his barbs, bringing up his rival's hands and skin tone.

Rubio struggled badly in the most recent rounds of primary contests. On Tuesday he failed to pick up a single delegate,  finishing fourth in Mississippi and Michigan behind Ohio Gov. John Kasich and earning just a single-digit percentage of the vote in both states.

Trump won three of the contests and Ted Cruz won one.

Rubio's weak standing has forced anxious Republican officials to come to terms with the idea that their second least favourite GOP presidential candidate, the polarizing Cruz, may be the party's best last chance to stop Trump.

Possible Cruz supporters include reluctant Senate colleagues and former presidential rivals with strong ties to major donors, who have long feared Cruz's purist ideology but dread the prospect of a Trump nomination even more. The first-term Texas senator on Wednesday announced the backing of one former primary opponent, Carly Fiorina, and is seeking the backing of another, Jeb Bush, on Thursday.

"It's an outsider year, and the most logical person to take on Trump based on past performance is Ted Cruz," said another former presidential opponent, South Carolina Sen. Lindsey Graham. Earlier in the year, Graham likened the choice between Cruz and Trump to "being shot or poisoned."

"He's not my preference," Graham said of Cruz. "But we are where we are. And if Trump wins Florida and Ohio, I don't know if we can stop him."

"I could see myself as a Cruz supporter," Nevada Sen. Dean Heller, a Rubio supporter, said Wednesday. "I agree with Cruz 80 per cent of the time. There are things I don't agree with him on, but if we are agreeing 80 per cent of the time and he's a conservative, we are going to get along just fine."

Still, no Republican senator has endorsed Cruz, who called Majority Leader Mitch McConnell a liar and helped engineer the 16-day partial government shutdown in 2013. Half a dozen Senate Republicans said Wednesday that they hadn't heard from either Trump or Cruz.

Trump has one Senate endorsement, from Alabama's Jeff Sessions, while Rubio has endorsements from 14 senators.

Bush meets with candidates ahead of Thursday debate

Fiorina, though, offered Cruz an immediate dose of credibility with his party's skeptical donor class. She told a Miami crowd on Wednesday that she's "horrified" by Trump.

"The truth is that Donald Trump and Hillary Clinton are two sides of the same coin," she charged, standing alongside Cruz. "It is time now to unite behind the one man who can beat Donald Trump, who can beat Hillary Clinton."

'The Republican establishment is in its death throes. The only remaining candidates are 100 per cent anti-establishment." - Mark Meckler, tea party leader

Not all think the choice is so clear cut.

"The Republican establishment is in its death throes," said longtime tea party leader Mark Meckler. "The only remaining candidates are 100 per cent anti-establishment."

Next Tuesday's winner-take-all contests in Florida and Ohio have injected a sense of urgency into the GOP's anti-Trump movement.

The billionaire businessman is calling on mainstream Republicans to unify behind his candidacy ahead of next week's primaries, which could give him an insurmountable delegate lead.

"If I win those two, I think it's over," Trump told CNN's Anderson Cooper's 360.

"Instead of fighting it, they should embrace it," he added on Fox News Channel.

Bush planned to confer with all the candidates — save Trump — ahead of Thursday's GOP debate. He met privately with Rubio on Wednesday, and planned to meet with Cruz and Kasich on Thursday.

Bush and Trump engaged in heated confrontations throughout Bush's campaign, repeatedly referring to each other as "loser."