“HOW DID you go bankrupt?” one character asks another in Ernest Hemingway’s “ The Sun Also Rises.” “Two ways,” is the response. “Gradually and then suddenly.”

In the matter of the Republican Party’s moral and ideological bankruptcy, the GOP is still in the “gradually” phase. Donald Trump is seemingly about to accumulate all, or nearly all, of the 1,237 delegates needed to win the GOP presidential nomination on the first ballot. In the face of this not quite entirely inevitable but quite entirely odious prospect, the thing to do is declare your unequivocal opposition and fight it. Alas, some leading Republicans, such as House Veterans’ Affairs Committee Chairman Jeff Miller (Fla.) and Transportation Committee Chairman Bill Shuster (Pa.), have endorsed Trump. Others are calibrating their responses, as politicians are wont to do.

These include Indiana Gov. Mike Pence, who leavened his endorsement of Sen. Ted Cruz (Tex.) with praise for Mr. Trump’s supposed tapping of legitimate voter concerns and a protestation that “I’m not against anybody.” The category also contains some people who should know better, such as Senate Foreign Relations Committee Chairman Bob Corker (Tenn.), who did not endorse Mr. Trump but observed, apropos the candidate’s unserious April 27 foreign policy address: “I think when somebody transitions and gives a serious speech about something, I think giving an ‘atta boy’ is an appropriate thing to do.”

And then there’s Sen. Marco Rubio (Fla.), last seen trading crude insults with Mr. Trump, en route to a humiliating primary defeat in his home state on March 15. In those days, Mr. Rubio choked up when asked if he’d keep his promise to support the GOP nominee, even if it should be Mr. Trump; he declared it was getting “harder every day” to do so. On April 20, however, it seemed to be getting easier again: “I’ve always said I’m going to support the Republican nominee, and that’s especially true now that it’s apparent that Hillary Clinton is going to be the Democratic candidate” — as if it hadn’t been apparent five weeks earlier, or as if Mr. Rubio would have been more sanguine at the prospect of Sen. Bernie Sanders (I-Vt.) in the White House. Though he added that his “differences” with Mr. Trump are “well documented,” he also allowed that the billionaire’s “performance has improved significantly,” as he put it on April 29.

Who knows why Republican politicians equivocate about the most repugnant political phenomenon in recent American history. Opportunism? Cluelessness? A sincere wish to influence the process for the better? Any of those, or a combination, would be preferable to a fourth alternative: actual approval of what Mr. Trump stands for — though plainly there is some of that, too.

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Someday, everyone involved in American politics will be called upon to account for his or her behavior during Mr. Trump’s run for the White House. The Republican Party chairman, Reince Priebus, forged one route months ago when he cheerfully pronounced: “Winning is the antidote to a lot of things.” It will be instructive to watch which politicians now follow Mr. Priebus to the moral poorhouse, and which have the gumption to chart a different course.