As Republican frontrunner Donald Trump moves ever-closer to securing the party's presidential nomination, members of the GOP establishment are pulling out all the stops to stand in his way.

Republican strategists are openly discussing all kinds of "anyone but Trump" strategies, including ways a dark horse candidate might be able to enter the race and steal the nomination away.

According to multiple U.S. political experts, two-time presidential candidate Mitt Romney might be the establishment's preferred dark horse candidate, though he has not announced his candidacy or endorsed anyone else. However, Romney did condemn Trump in a speech at the University of Utah, calling him a "con man" and a "fake."

But if the GOP hopes to put forward a pro-establishment candidate, it will need to prevent Trump from securing too many states over the next two weeks.

Conservative strategist Amy Kremer says Trump could be stopped if he falls short of winning the 1,237 delegates needed for the "winner-take-all" round of voting, in mid-March. If Trump doesn't enter that round with enough votes to win the nomination in the first round of voting, other candidates would be free to "horse-trade" for votes in order to unseat the frontrunner.

That situation – called a contested, or brokered convention – would allow the GOP to put forward a new candidate, if it chose to do so. "Talk is that would be Mitt Romney," Kremer told CTV's Canada AM.

Mary Kate Cary, a former speechwriter for President George H. W. Bush, says the Republicans will want to keep as many candidates in the current race as possible, in order to keep Trump from securing 1,237 delegates.

Marco Rubio, for instance, could take a big bite out of Trump's fortunes if he manages to win in Florida, Cary said.

"That might keep Donald Trump from getting to (1,237)," Cary told CTV News Channel.

Kremer pointed out that many Republican fundraisers are pouring their money into anti-Trump ads in Florida, to keep him from winning that state.

Romney has not officially entered the race, but he did come out publicly against Trump on Thursday. Romney attacked Trump on his behaviour, his business record, his policies and his view of the world.

"His imagination must not be married to real power," Romney said. The former presidential candidate also predicted Trump would answer back with "every low-road insult" he could muster.

Trump went on the offensive before Romney's speech, with a string of attacks on Twitter.

Kremer says Trump's popularity is rooted in a general sense of dissatisfaction among U.S. voters, and a desire for "outsider" candidates who are not part of the political world.

"Donald Trump is just emblematic of the anger and frustration with the American voters across the country," Kremer said, adding that the frontrunner could have been "anyone else" with the same "outsider" status. Texas Senator Ted Cruz, for instance, could be considered another outsider, Kremer said.

Cary suggested Romney's presence won't do much to sway that "outsider" vote back toward the GOP establishment. Instead, Romney's words will only hold sway with those already in favour of the pro-establishment candidates.

"Trump supporters are probably not Romney fans, so I think there is a bit of preaching to the choir," she said.

She added that Romney or Paul Ryan, the Republican Speaker of the House, might have a chance to defeat Trump if one of them ran as a third-party candidate.