"Have you no sense of decency, sir, at long last?"

That's the question Joseph N. Welch, special counsel to the U.S. Army, asked of Sen. Joe McCarthy in 1954. It helped destroy the Wisconsin lawmaker, and end his red-baiting, witch-hunting career.

Now it's time for all Americans, especially Republicans running for president, to say the same thing to Donald Trump. His latest despicable proposal, a ban on all Muslims entering this country, puts him in the same class as McCarthy, a bigoted bully preying on the worst strains of American nativism.

The shameful stain of McCarthyism taught us a clear lesson: bullies won't stop until courageous figures stand up to them. Republicans who want to lead this country have to follow Welch's example, forcefully condemning Trump's words and promising -- with no reservations or escape clauses -- to withhold their support if he wins the Republican nomination.

Ohio Gov. John Kasich set a good example by forcefully condemning Trump: "This is just more of the outrageous divisiveness that characterizes his every breath and another reason why he is entirely unsuited to lead the United States."

Others like Sen. Ted Cruz, who decline to confront Trump, are condoning him. History will mark them as cynical cowards.

But Trump's crusade against Islam echoes far beyond the campaign trail. It raises a critical question: How will this country, and the next president, grapple with the threat of global terrorism?

To put it bluntly: Trump is the Islamic State's best ally. His overwrought, wrongheaded rhetoric makes any leader's job much more difficult.

As President Obama put it in his address to the nation: "We cannot turn against one another by letting this fight be defined as a war between America and Islam. That, too, is what groups like ISIL (an acronym for Islamic State) want. ISIL does not speak for Islam. They are thugs and killers, part of a cult of death, and they account for a tiny fraction of more than a billion Muslims around the world -- including millions of patriotic Muslim-Americans who reject their hateful ideology."

That's true. But it's not just time for Republicans to stand up to Trump. It's time for Muslims to stand up to ISIL.

The "millions of patriotic Muslim-Americans" who reject extremism have to make their voices heard -- loudly and clearly. They have to be part of the solution, not part of the problem.

One part of the president's speech that deserves more attention was his emphasis on the role Muslims need to play in the battle against jihadism. "If we're to succeed in defeating terrorism," he said, "we must enlist Muslim communities as some of our strongest allies, rather than push them away through suspicion and hate.

"That does not mean denying the fact that an extremist ideology has spread within some Muslim communities," he added pointedly. "This is a real problem that Muslims must confront, without excuse."

Yes, they must, and it's not just an abstract question of proving their loyalty and patriotism. Muslims can play two very practical roles in combatting "extremist ideology."

The first is on the battlefield of ideas. The jihadists of ISIL are spreading their "poisonous narrative" says FBI director James Comey "in a slick way through social media ... 24 hours a day."

There has to be concerted counterattack, a counter-narrative led by Muslim leaders who preach the virtues of peaceful integration into American society.

As Lisa Monaco, the president's counterterrorism adviser, told The New York Times: "We can work with the private sector to get additional messengers with alternative voices out there. Frankly, we've got to do a better job of approaching this in a way that allows us to -- the phrase has been used -- break the brand of ISIL's message."

There's a second role for Muslims as well: intelligence agents. The shooters in San Bernardino, California, reflect a dangerous new trend: homegrown warriors who are far harder to trace and track than foreign agents.

Friends and relatives, pastors and parishioners -- they all have to feel responsible for spotting the potential terrorist next door. Homeland Security chief Jeh Johnson has been touring the country, speaking to Muslim communities, and as he told the Times: "The overarching message to them is, help us help you -- help us to identify someone in your community who may be heading in the wrong direction."

As the president says: No more excuses. No excuses for Republicans who won't confront Trump. No excuses for Muslims who won't confront ISIL. Our deepest values are at stake.