Recognizing that Mr. Trump has seized a formidable advantage in the race, they say that an effort to block him would rely on an array of desperation measures, the political equivalent of guerrilla fighting.
There is no longer room for error or delay, the anti-Trump forces say, and without a flawlessly executed plan of attack, he could well become unstoppable.
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 last Tuesday in Florida and three other states.
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.
Mr. 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,” Mr. Coburn said, “and I don’t expect that person to be me.”
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 Gov. John Kasich of Ohio.
Mr. Trump has a delegate lead of about 250 over Mr. 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 D. 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.
David McIntosh, president of the conservative Club for Growth, which has spent millions on ads attacking Mr. Trump, said his group met on 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 Delegate War
Central to this plan is stopping Mr. Trump in Wisconsin, the next major showdown after contests that Mr. Trump and Mr. 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 $2 million in Wisconsin and arguing that “the only viable option to defeat Donald Trump is Ted Cruz.”
The memo conceded it was “very unlikely” that Mr. Cruz could overtake Mr. Trump in the delegate count, but outlined a strategy to deny Mr. Trump the 1,237 delegates required to clinch the nomination before the convention in Cleveland in July.
Mr. Cruz and Mr. Kasich also see the Wisconsin primary as pivotal. Mr. 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.
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 strong. A few of the remaining winner-take-all states, like Montana and South Dakota, appear friendly to Mr. Cruz.
Anti-Trump Republicans said they would use the six weeks between the last primaries and the mid-July convention to woo individual delegates.
A number of states, including Pennsylvania and Colorado, send large numbers of uncommitted delegates to the convention, making those party regulars especially ripe targets for courtship. Other states will be sending delegates bound to candidates who have left the race, like Senator Marco Rubio of Florida and Jeb Bush. Those delegates could be persuaded to vote for Mr. Cruz or Mr. Kasich after the first ballot.
To justify rejecting Mr. Trump in Cleveland, Republicans say they will have to convince both delegates and the public that it was not the party’s obligation to hand him a nomination he did not secure on his own.
“The burden is on Trump, not the party, if he fails to clinch the nomination,” said David Winston, a Republican pollster who advises the House leadership. “He has presented himself as the ultimate dealmaker, and it’s on him to close this one.”
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.
A Split Opposition
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 favor Mr. Cruz, while many Republican elites have warmed to Mr. 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 Mr. 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 Mr. Cruz runs deep in Washington. Since the withdrawal of Mr. Rubio, who had the support of many fellow senators, just one has endorsed Mr. Cruz.
About two dozen conservative leaders met Thursday at a private club in Washington, where some pushed for the group to come out for Mr. 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 Mr. Kasich and Mr. Cruz to avoid competing in states where one of them is favored. “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 détente, Mr. Kasich ran ads and campaigned in Utah this weekend, angering aides to Mr. Cruz, who hopes to reach the 50 percent threshold needed to claim all the state’s delegates. Mr. Kasich also refused to participate in a one-on-one debate, without Mr. Trump — denying them both considerable media exposure and an important online fund-raising opportunity.
But Mr. Kasich’s backers have no appetite for a head-to-head ideological fight with Mr. Cruz on national television. They are focused on winning delegates wherever they can so that nobody reaches a majority before Cleveland, where Mr. Kasich’s supporters plan to argue that he is the only electable Republican contender.
Abandoning the Party
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 R. Bloomberg, the former New York City mayor who considered an independent run, concluded that petition gathering would have 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 already 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?”
At the meeting in Washington Thursday, Mr. Perry’s name was raised among several options, but his associates said that while he has fielded inquiries, he remains committed to backing Mr. Cruz.
Mr. Erickson described Mr. Perry as a “consensus choice” for an independent campaign: “He would win Texas and at least obstruct Trump.”
Nicholas Sarwark, chairman of the national Libertarian Party’s executive body, said its convention in Orlando could be open to a late entrant. But several candidates are already pursuing the nomination, including Gary Johnson, the former New Mexico governor who was the party’s 2012 nominee.
“Theoretically, someone could come in,” Mr. Sarwark said, adding, “But you’d better be a pretty impressive man off the street.”
Author: ALEXANDER BURNS and JONATHAN MARTIN