Chris Christie was asked Sunday evening about presidential rival Donald Trump’s nonsense claim, reviving an Internet conspiracy theory, that thousands of Muslims in New Jersey cheered the World Trade Center’s collapse in 2001.

The New Jersey governor’s reply was a paean to pusillanimity.

“I do not remember that, and so it’s not something that was part of my recollection,” he said. “I think if it had happened, I would remember it. But, you know, there could be things I forget, too.”

Presented with a malicious lie by the front-runner for the Republican presidential nomination, Christie courageously raised doubts – about his own memory.

Republican elites are panicky about the durable dominance of Trump (and to a lesser extent Ben Carson) in the presidential race. They are right to worry, but Trump is a problem of their own creation.

Trump gets ever more base in his bigotry – and yet, with few and intermittent exceptions, rival candidates, party leaders and GOP lawmakers decline to call him out. So he continues to rise, benefiting from tacit acceptance of his intolerance.

Or more than tacit. Carson, taking questions from reporters Monday afternoon, said that he, too, had seen nonexistent “newsreels” of the supposed cheering by New Jersey Muslims on 9/11. (His spokesman said later that Carson had been mistaken.)

For months – years, really – Republicans have averted their gaze from Trump’s attacks on women, Hispanics and immigrants. Now the racism becomes more overt – and still, he goes unchallenged.

On Saturday, a black demonstrator at a Trump rally in Birmingham, Alabama, was kicked and punched by white men in the crowd after he fell to the ground and one of the men put his hands around the demonstrator’s neck as if to choke him. Trump’s response? Maybe the man “should have been roughed up,” he said Sunday.

A few hours later, Trump re-tweeted a graphic with invented statistics showing, falsely, that black people are responsible for most killings of white people. Disseminating this bogus graphic, which appears to have originated with neo-Nazis, followed a week in which Trump talked about forcing American Muslims to be registered in a database, putting mosques under surveillance and possibly closing them.

Yet no matter how far Trump goes, most of his competitors stay silent, or mild, or deferential.

When Trump talked about registering Muslims – a proposal that has Nazi echoes – the response was tame. Jeb Bush called the idea “abhorrent,” but he had just tried to out-Trump Trump by suggesting that only Christian refugees from Syria should be admitted. Carson said it would be a “dangerous precedent,” but he’s on record saying a Muslim shouldn’t be president and comparing some refugees to “rabid dogs.” Ted Cruz let it be known that “I’m not a fan of government registries of American citizens” but preceded this with “I’m a big fan of Donald Trump’s.”

Even when Trump left unchallenged a supporter at his campaign event who called Obama a Muslim and said Muslims are “a problem in this country,” most rivals declined to criticize Trump – who RNC Chairman Reince Priebus has called a “net positive” for the party.

Contrast that with the Democrats’ handling of David Bowers, the Democratic mayor of Roanoke, Virginia, who last week invoked the internment of Japanese-Americans in his call to suspend the settlement of Syrian refugees in his area. Bowers was immediately booted from his spot on Hillary Clinton’s Virginia leadership team.

But the front-runner for the Republican presidential nomination talks about the forced registration of Muslims. Republican leaders look away. And Trump surges in the polls, regaining the lead he had lost before the Paris attacks. For Republican leaders and rival candidates, these are the wages of cowardice.

Two months ago, after the second GOP debate, I saw signs that Republican hopefuls had begun to think it safe to take on Trump “consistently and jointly.” But that didn’t continue. There’s no incentive for an individual Republican candidate to take on Trump only to get mowed down by his counterassault. Instead, some rivals are imitating Trump’s positions.

Republican officials say the fear of challenging Trump won’t subside unless a credible alternative to him emerges.

But that will require more candidates to quit the race, and Republicans are running out of time. Voting begins in Iowa on Feb. 1, and, a month later, 16 states will have voted. By the end of March, 64 percent of Republican delegates will have been awarded.

The longer Republican leaders take to find their anti-Trump voices, the more their quiescence becomes an endorsement.

Dana Milbank is a columnist for The Washington Post. He can be contacted at:

danamilbank@washpost.com