The Republicans who dominate the right-leaning magazines, journals and political groups can live with Mr. Cruz, believing that his nomination would leave the party divided, but manageably so, extending a longstanding intramural debate over pragmatism versus purity that has been waged since the days of Barry Goldwater and Nelson Rockefeller. They say Mr. Trump, on the other hand, poses the most serious peril to the conservative movement since the 1950s-era far-right John Birch Society.

Rich Lowry, the editor of National Review — embracing the role of his predecessor, William F. Buckley, who in the 1960s confronted Birch Society members — reached out to conservative thinkers to lend their names to the manifesto against Mr. Trump. He drew on some of the country’s leading conservatives, including Erick Erickson, William Kristol and Yuval Levin, to write essays buttressing the argument that Mr. Trump had no commitment to restraining the role of government and possessed authoritarian impulses antithetical to conservative principles.

“Donald Trump is a menace to American conservatism who would take the work of generations and trample it underfoot on behalf of a populism as heedless and crude as The Donald himself,” the magazine said in an editorial accompanying the manifesto, titled “Against Trump.”

Peter Wehner, a longtime conservative writer, said: “He’s not a conservative, he’s an angry populist. It would be dangerous if the party or movement hands control over to him.”