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,...