Republikanernas kandidat 2016?

Republican leaders adamantly opposed to Donald J. Trump’s candidacy are preparing a 100-day campaign to deny him the presidential nomination, starting with an aggressive battle in Wisconsin’s April 5 primary and extending into the summer, with a delegate-by-delegate lobbying effort that would cast Mr. Trump as a calamitous choice for the general election.

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.

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,” ...

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.

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...

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.