Could this be the beginning of the end of Donald Trump?

Last week’s debates capped several days in which the candidates for the Republican presidential nomination have fundamentally changed their tone toward the nominal front-runner. They are, at long last, treating him like the huckster he is.

Scott Walker, who cited Trump entities’ bankruptcy filings, said: “Mr. Trump, we don’t need an apprentice in the White House.”

George Pataki: “Donald Trump is unfit to be president of the United States.”

Rand Paul condemned Trump’s “sophomoric” attacks on personal appearances, saying we should “all be worried to have someone like that in charge of the nuclear arsenal.” (Trump responded with a joke about Paul’s looks.)

The Republican presidential contenders had a lot to say about Donald Trump at the GOP debate. (CNN)

Bobby Jindal, asked about his recent description of Trump as an “unstable, narcissistic egomaniac,” predicted: “God forbid, if he were in the White House, we have no idea what he would do.”

Even genial Jeb Bush demanded Trump apologize for the “completely inappropriate” way the mogul referred to Bush’s Mexican-born wife. And Carly Fiorina likened Trump to the federal government, saying he “ran up mountains of debt, as well as losses, using other people’s money.”

Variations of these lines had been used before, but the cumulative impact indicates Republican hopefuls now think it safe to take on the bombastic billionaire. That’s an important development and could indicate rising surface tension in the Trump bubble. Trump can beat anybody in one-on-one insult spewing, but if his rivals — and conservative thought leaders generally — take him on consistently and jointly, voters will read their cues and Trump’s moment will end.

Trump’s rivals continued to challenge him on Friday, after the businessman let stand an anti-Muslim rant from a questioner at one of his events who also called President Obama a Muslim. Lindsey Graham accused Trump of “playing into this hateful narrative,” while Chris Christie said Trump had an “obligation” to set the record straight. Various prominent conservatives joined in the condemnation.

Four years ago, when Trump led the “birther” movement, Obama called him a “carnival barker.” Conservatives have embraced that view; Jindal this month called Trump a “carnival act.” And that could be the tycoon’s undoing. People will disagree about his politics, but once the perception spreads across ideological lines that Trump is a clown, he’s done.

Trump helps the perception by sounding buffoonish. “I say, not in a braggadocious way, I’ve made billions,” he announced Wednesday. “I’m number one in every polls [sic] by a lot,” he further declared, and at another point asserted that “I’m a very militaristic person.” Asked about a Hugh Hewitt radio interview in which he confused the Kurds with the Iranian Quds Force and seemed not to recognize the name of Quds Force leader Qasem Soleimani, Trump explained that “Hugh was giving me name after name, Arab name, Arab name.” (Soleimani is not an Arab.)

Trump’s defense of his foreign policy ignorance? “I will know more about the problems of this world by the time I sit, and you look at what’s going on in this world right now by people that supposedly know, this world is a mess.” Trump’s clownish facial expressions of mockery and protest, caught on split screen while others spoke, did not help him convey gravitas. Neither did his juvenile taunts when Bush spoke about denying Trump casino gambling in Florida even though Trump hosted a fundraiser for Bush: “Totally false . . . Wrong! Don’t make things up, Jeb.” (Bush’s version was correct.)

Probably the most encouraging development was the candidates’ willingness to support each other in calling out Trump’s boorishness. At one point, Trump hectored Bush about his retracted claim that too much is spent on women’s health: “I heard it myself. Why did you say it?” A moment later, when Carly Fiorina was asked about Trump’s disparagement of her looks, which he later said was about her “persona,” Fiorina replied: “Mr. Trump said that he heard Mr. Bush very clearly and what Mr. Bush said. I think women all over this country heard very clearly what Mr. Trump said.”

Without fear of retribution, the candidates played tag-team against the bully, scolding him for “careless language” (Paul), “using the talking points of the Democrats” (Walker), his plan to “tear families apart” with mass deportation (Bush), his birthright-citizenship plan to “just wave your hands and say the 14th Amendment is going to go away” (Fiorina) and his defiance of the “extremely well-documented proof that there’s no autism associated with vaccinations” (Carson).

Pressed about his criticism of Bush for speaking Spanish, Trump lost some of his bluster, saying he made the remark “a little bit half-heartedly.”

Even this flicker of remorse is a sign of progress. It raises hope that Trump will indeed succeed in making America great again — by motivating Americans, even fellow conservatives and Republicans, to repudiate his nonsense.

Twitter: @Milbank

Read more from Dana Milbank’s archive, follow him on Twitter or subscribe to his updates on Facebook.