The implosion over Donald Trump’s candidacy that Republicans had hoped to avoid arrived so virulently this weekend that many party leaders vowed never to back the billionaire and openly questioned whether the GOP could come together this election year…

“This is an existential choice,” said former senator Norm Coleman of Minnesota, who is backing Rubio. Asked how the party could unite, Coleman said: “It gets harder every day when you hear things like not disavowing the KKK and David Duke. It’s not getting easier; it’s getting more difficult. . . . I’m hopeful the party won’t destroy itself.”…

“For many Republicans, Trump is more than just a political choice,” said Kevin Madden, a veteran operative who advised 2012 nominee Mitt Romney. “It’s a litmus test for character.”

Madden, like some of his peers, said he could never vote for Trump. If he is the nominee, Madden said, “I’m prepared to write somebody in so that I have a clear conscience.”