From left, John Kasich, Carly Fiorina, Marco Rubio, Ben Carson, Donald Trump, Ted Cruz, Jeb Bush, Chris Christie and Rand Paul take the stage at the CNN Republican presidential debate in Las Vegas. (Mark J. Terrill/Associated Press)

THE REPUBLICAN Party, once small government’s champion, is now the party that breeds presidential contenders who would monitor schools and mosques, shut down parts of the Internet and exclude certain immigrants for no reason beyond the faith they profess. In the GOP debate Tuesday, those ideas — along with can-you-top-this rhetorical barrages aimed at illegal immigrants and Syrian refugees — received a generally polite reception, with constitutional, legal and practical questions contemptuously dismissed as “political correctness.”

True, the extremism that now passes for mainstream Republican thought, robbed of its shock value by the unfiltered ravings of Donald Trump, was punctured from time to time with expressions of dismay, incredulity and doubt.

Former Florida governor Jeb Bush and Sen. Rand Paul (Ky.) skewered Mr. Trump’s plans for the United States to ban all Muslim immigrants or murder the families of terrorists, and Mr. Paul, along with Ohio Gov. John Kasich, rightly dismissed Mr. Trump’s blithe suggestion that he could somehow censor the Internet in parts of the world where jihadist sentiment runs deep.

By and large, though, these ludicrous proposals went unremarked on by the Republican contestants, for whom bigotry, hatred and magical thinking are the new normal.

The GOP’s ideological sands are shifting with whiplash-inducing speed. Just a week after House Speaker Paul D. Ryan (R-Wis.) denounced Mr. Trump’s call to bar entry to any Muslim immigrant, Sens. Marco Rubio (Fla.) and Ted Cruz (Tex.) both said they could “understand” the impulse behind the patently un-American idea, although each politely disagreed. As for Mr. Trump’s four other main rivals, who flanked him on the stage in Las Vegas, none bothered even to address what surely counts as one of the most incendiary proposals ever made by a candidate seeking a major party’s presidential nomination.

It could be that the candidates quail at contending with the question of banning Muslims because polls suggest that about 60 percent of GOP primary voters like the idea. (A roughly equal proportion of all Americans don’t.) However, lunacy has always had a constituency in this country — plenty of people think the moon landing was a hoax and that the Sept. 11, 2001, attacks were an inside job. A litmus test for presidential candidates is whether they have the spine to speak truth to the fringe. By that standard, most of the current crop of Republican hopefuls fail.

The candidates were no more courageous on the question of admitting Syrian refugees victimized by a dictatorial regime and the Islamic State’s death cult. Despite the fact that neither the San Bernardino, Calif., assailants nor any of the known Paris attackers appears to have been Syrian — most in Paris were French nationals — virtually all the GOP contestants jockeyed to vilify Syrian refugees, with Mr. Trump raising the fact-free specter of “tens of thousands of [refugees] having cellphones with ISIS [Islamic State] flags on them.”

Fear-mongering and raw xenophobia were once the hallmarks of fringe candidates. Today the fringe candidates have stormed center stage, brandishing their zeal and hyperbole and, disturbingly, dragging the mainstream along with them.

Here are highlights from CNN's Republican presidential debate on Dec. 15 in less than two minutes. (Sarah Parnass/The Washington Post)