But should that effort falter, leading conservatives are prepared to field an independent candidate in the general election, to defend Republican principles and offer traditional conservatives an alternative to Mr Trump's hard-edged populism. They described their plans in interviews after Mr Trump's victories on Tuesday in Florida and three other states. Republican presidential candidate Donald Trump campaigning in Ohio earlier this month. Credit:AP The names of a few well-known conservatives have been offered up in recent days as potential third-party standard-bearers, and William Kristol, editor of The Weekly Standard, has circulated a memo to a small number of conservative allies detailing the process by which an independent candidate could get on general-election ballots across the country. Among the recruits under discussion are Tom Coburn, a former Oklahoma senator who has told associates that he would be open to running, and Rick Perry, the former Texas governor who was suggested as a possible third-party candidate at a meeting of conservative activists on Thursday in Washington. Dr Coburn, who left the Senate early last year to receive treatment for cancer, said in an interview that Mr Trump "needs to be stopped" and that he expected to back an independent candidate against him. He said he had little appetite for a campaign of his own, but did not flatly rule one out.

"I'm going to support that person," Dr Coburn said, "and I don't expect that person to be me." A protester with an anti-Donald Trump sign in Tucson, Arizona, on Saturday. Credit:AP Mr Trump opponents convened a series of war councils last week to pinpoint his biggest vulnerabilities and consider whether to endorse one of his two remaining opponents, Senator Ted Cruz of Texas and Governor John Kasich of Ohio. Mr Trump has a delegate lead of about 250 over Senator Cruz, the second-place candidate, but he has repeatedly acted in ways that push party leaders farther from his camp. On Thursday, House Speaker Paul Ryan sternly admonished him for saying his supporters would riot if Republicans nominated someone else, the latest in a series of remarks Mr Trump has made that seemed to encourage or condone violence. Protesters mix with Trump supporters Republican in Tucson, Arizona, on Saturday. Credit:AP

David McIntosh, president of the conservative Club for Growth, which has spent millions on ads attacking Mr Trump, said his group met last Wednesday and concluded it was still possible to avert Mr Trump's nomination. The group plans a comprehensive study of Trump supporters to sharpen a message aimed at driving them away from him. "This is still a winnable race for a free-market conservative that's not Donald Trump," Mr McIntosh said, adding, "It's not a layup, but there's a clear path to victory." A supporter for Democratic presidential candidate Senator Bernie Sanders, gives the thumbs down sign to a fellow Sanders' supporter wearing a Donald Trump mask at a rally in Phoenix, Arizona, on Saturday. Credit:AP Central to this plan is stopping Mr Trump in Wisconsin, the next major showdown after contests that Mr Trump and Senator Cruz are expected to split this week in Arizona and Utah. On Thursday, the Club for Growth sent a three-page memo to influential Republican donors promising to spend as much as $US2 million ($2.6 million) in Wisconsin and arguing that "the only viable option to defeat Donald Trump is Ted Cruz".

Protesters shout as they are removed from the venue in Tucson on Saturday as Republican presidential candidate Donald Trump speaks. Credit:AP The memo conceded it was "very unlikely" that Senator Cruz could overtake Mr Trump in the delegate count, but outlined a strategy to deny Mr Trump the 1237 delegates required to clinch the nomination before the convention in Cleveland in July. Senator Cruz and Governor Kasich also see the Wisconsin primary as pivotal. Senator Cruz's campaign is dispatching additional staff members there and opening a "Camp Cruz" to house volunteers. The campaign will begin running ads there in the next few days, aiming to get a head start on Mr Trump in the state. A Donald Trump protester and supporter in an altercation at the conclusion of a Trump rally at the Tucson Convention Centre on Saturday. Credit:AP Beginning with Wisconsin, the race moves into states that apportion delegates based on who wins in each congressional district, which would allow anti-Trump forces to peel delegates away from him in states like New York and California, where he is expected to run strongly. A few of the remaining winner-take-all states, like Montana and South Dakota, appear friendly to Senator Cruz.

Mr Trump has said that he expects to win a majority of the delegates before Cleveland, and that if he falls just short it would be unconscionable for the party to nominate someone else. Mr Trump's hand has been strengthened by disagreements within the stop-Trump forces, which fall along familiar lines: Conservative activists are uneasy with the party establishment and favour Senator Cruz, while many Republican elites have warmed to Governor Kasich, recoiling from those they perceive as ideological purists. Mitt Romney, the party's nominee in 2012, attempted to bridge that divide on Friday by revealing that he would support Senator Cruz in Utah and warning that "a vote for Governor Kasich in future contests makes it extremely likely that Trump-ism would prevail". But contempt for Senator Cruz runs deep in Washington. Since the withdrawal of Senator Rubio, who had the support of many fellow senators, just one has endorsed Senator Cruz. About two dozen conservative leaders met on Thursday at a private club in Washington, where some pushed for the group to come out for Senator Cruz to rebut the perception that the stop-Trump campaign was an establishment plot.

"If we leave here supporting Cruz, then we're anti-establishment," said one participant, who could be heard by a reporter outside. But the group failed to agree on an endorsement, instead pleading for Governor Kasich and Senator Cruz to avoid competing in states where one of them is favoured. "They're going to have to come to terms and lay off each other," said Erick Erickson, an influential conservative commentator, who convened the meeting. Yet in a sign that there is no such detente, Governor Kasich ran ads and campaigned in Utah this weekend, angering aides to Senator Cruz, who hopes to reach the 50 per cent threshold needed to claim all the state's delegates. Governor Kasich also refused to participate in a one-on-one debate, without Mr Trump - denying them both considerable media exposure and an important online fundraising opportunity. But Governor Kasich's backers have no appetite for a head-to-head ideological fight with Senator Cruz on national television. They are focused on winning delegates wherever they can so that nobody reaches a majority before Cleveland, where Governor Kasich's supporters plan to argue that he is the only electable Republican contender.

For Republicans opposed to Mr Trump under any circumstances, a third-party campaign offers a last refuge. Such a candidacy might gain support from high levels of the party: Mr Romney has said he would be inclined to vote for a third candidate over Mr Trump and Hillary Clinton. Advisers to Michael Bloomberg, the former New York City mayor who considered an independent run, concluded that petition gathering would have had to begin by early March for a candidate to appear on November ballots in all 50 states. But an independent could still get on ballots in dozens of states - or perhaps seek the nomination of the Libertarian Party, which is on the ballot in most states and does not pick a candidate until late May. Mr Kristol, a leading critic of Mr Trump, said in an interview that he believed it was not too late to put forward a viable independent candidacy. "I think the ballot access question is manageable," he said. "The big question is, who's the candidate?"

New York Times