Holy Mitt, what a meltdown.

Add this one to Donald Trump’s lengthening list of firsts: He’s forced a Republican Party reckoning overdue for years, all in a few days. It took the Trump-dominated Super Tuesday contests to awaken Republican leaders to the fact that the darkest elements of the party’s base, which many of them have embraced or exploited, are now threatening their party.

Last week, Marco Rubio and Ted Cruz, elected to the Senate partly on their appeal to extremists, seemed to realize that they weren’t attractive enough to win Mr. Trump’s crowd. Just in time for Super Tuesday, they could see that ignoring or cozying up to Mr. Trump wasn’t working, and began attacks that have so far done nothing to slow his march. They were then joined by the G.O.P. campaign money machinery, which this week began frantically tossing more millions at — what? An anti-Trump ad campaign? A third-party effort?

Then came an open letter from 95 Republican national security experts, who declared themselves “united in our opposition to a Donald Trump presidency.” Of Mr. Trump they wrote: “He swings from isolationism to military adventurism within the space of one sentence.” Yet some among them have swung wildly in those same directions. Some were Bush administration officials who supported some of the worst foreign policy disasters this country has ever experienced, including the Iraq war. It is rich that they should now criticize Mr. Trump for policies that could make America less safe.

Then, on Thursday morning came Mitt Romney’s rambling indictment of Mr. Trump. After months of silence, Mr. Romney spent 20 minutes calling Mr. Trump a fraud and a phony with a record of business failures, whose economic ideas would put the United States into recession and whose foreign policy approach would endanger Americans.