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, and Republicans are now stuck with a collective-action problem. 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.