Insult after insult flew during the Fox News GOP debate on March 3. Here’s a look at some of the choice words rivals Marco Rubio, Ted Cruz and Donald Trump had for each other, while John Kasich stayed out of the fray. (Jenny Starrs/The Washington Post)

Only a year ago, Republicans were congratulating themselves on having the strongest field of presidential candidates in a generation — diverse, highly credentialed conservatives who might be the salvation of a party that had lost the popular vote in five of the past six elections.

But now, the question is how close the Grand Old Party will come to annihilating itself and what it stands for.

Donald Trump — dismissed by GOP elders for months as an entertaining fringe figure who would self-destruct — has staged a hostile takeover and rebranded the party in his own image. What is being left by the wayside is any sense of a Republican vision for the country or a set of shared principles that could carry the party forward.

A substance-free shout-fest billed as a presidential debate Thursday night marked a new low in a campaign that has seen more than its share of them.

The increasingly prohibitive front-runner and his three remaining opponents spent nearly the entire two hours hurling insults back and forth, with Trump at one point making a reference to the size of his genitalia.

“My party is committing suicide on national television,” tweeted Jamie Johnson, an Iowa political operative who had been an adviser to former Texas governor Rick Perry, one of the dozen Republicans whose presidential campaigns have been incinerated by the Trump phenomenon. The latest, retired neurosurgeon Ben Carson, formally dropped out Friday.

Also Friday, Trump clarified earlier statements that as president, he would order the U.S. military to waterboard militants and carry out other acts that violate international law.

In a statement, he said he understands “that the United States is bound by laws and treaties and I will not order our military or other officials to violate those laws and will seek their advice on such matters.”

In Thursday’s debate, moderator Bret Baier had asked Trump what he would do if service members refused to comply with his orders for exteme measures. The candidate replied, “If I say do it, they’re going to do it. That’s what leadership is all about.”

Trump’s musings on torture were among the many remarks that have alarmed establishment Republicans as worrisome and reckless.

“Republicans in general tend to be a group of people who like to view themselves as serious, having decorum, being orderly, being thoughtful,” said Roger Porter, who served as a senior policy official in the White Houses of Ronald Reagan and George H.W. Bush and who is now a professor at Harvard University’s John F. Kennedy School of Government.

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But, he said, Thursday’s debate “was the culmination of a long process of the people running for president this year finding themselves drug into a maelstrom in which they look anything but serious and calm and thoughtful and responsible. That’s very distressing for most Republicans. How did we get to this situation?”

More urgent, many Republicans say, is the question of how they get out of it.

Part of the decision is how to handle Trump himself.

Republican leaders are divided. Some are focusing their efforts on stopping the billionaire celebrity, even if it means overturning the will of GOP voters at the July convention in Cleveland. Others are arguing that they should coalesce behind him, so that Republicans will have their best chance this fall of beating Hillary Clinton, the Democratic front-runner, who is not without her own vulnerabilities.

Beyond that, some worry that even as Trump is bringing record numbers to the polls in the primary race, he is changing the very identity of the party. He is a new kind of Republican, one who flaunts his apostasies on conservative principles, who slings vulgar and divisive language, and who has an ostentatious disregard for the system.

All of which captures a current in the electorate. “The main pendulum in American politics is no longer swinging from left to right. It’s swinging between insiders and outsiders,” said Sen. David Perdue (R-Ga.). “It’s those in the political class against those who are not — that’s the divide in the country, in the party.”

Arthur Brooks, an independent who heads the conservative American Enterprise Institute think tank, said that “this is completely predictable, given where we are in the recovery from our financial crisis.”

“Financial crises take 15 to 20 years to clear, as a historical matter, and after two or three years, wealthy people have recovered, but working people haven’t,” he said. “So the result is they turn to populist solutions, and that’s exactly what we’re seeing.”

The GOP has always had internal tensions, but they have generally been over ideology — pitting its internationalists against its more libertarian non-interventionists on foreign policy, or its supply-siders vs. its deficit hawks on fiscal issues.

“What is happening now is bigger and less remediable in part because the battles in the past were over conservatism, an actual political philosophy,” Wall Street Journal columnist Peggy Noonan, a former Reagan speechwriter, wrote this week.

“We are witnessing history. Something important is ending,” she added.

During the debate, Sen. Marco Rubio (Fla.) insisted: “We are not going to turn over the conservative movement, or the party of Lincoln or Reagan, for example, to someone whose positions are not conservative. To someone who last week defended Planned Parenthood for 30 seconds [on] a debate stage. To someone, for example, that has no ideas on foreign [policy] — someone who thinks the nuclear triad is a rock band from the 1980s.”

Nonetheless, Rubio said he will support Trump if the mogul is the party’s nominee. So did the other two Republican candidates, Sen. Ted Cruz (Tex.) and Gov. John Kasich (Ohio).

That is in part because the party put itself in handcuffs on that question in September, when its leaders were terrified that Trump would bolt and run as an independent. He signed an oath to support whoever wins the nomination.

Now, it is arguable that Republicans would be better off if Trump had launched a third-party bid. Presumably, he would have been training most of his fire on Clinton, rather than the Republicans he mocks as “Little Marco” and “Lyin’ Ted.”

Still, Republicans are closely divided on the impact that Trump is having on their party’s image. In a December Marist poll for MSNBC and Telemundo, 43 percent of those surveyed said he is helping the GOP brand, while 40 percent said he is hurting it.

However, the numbers showed a negative trend from the same poll three months earlier, in which 48 percent said Trump was an asset to the party’s image and 35 percent said he was damaging it.

Where Noonan and others see the Trump phenomenon as a sea change for the GOP, Porter predicted that it will be transitory. He noted that the party went through a somewhat parallel identity crisis in 1940, when it nominated businessman Wendell Willkie, who only a year before was a registered Democrat.

Four years later, it turned back to a conventional Republican, New York Gov. Thomas Dewey.

“I don’t see this as having that big of a long-term effect, because I think it is sui generis to Donald Trump,” Porter said. “It’s very big in the short run. I don’t think it’s very big over the long run, because people have very short memories.”

Robert Costa and Emily Guskin contributed to this report.