Sen. John McCain tells the Washington Post he'd support Trump if he were to become the GOP's nominee for president, while Sen. Jeff Flake says there's no way Trump will win the nomination:

The response from the GOP leaders illustrates the tricky position in which Republican leaders find themselves when it comes to the unpredictable Trump. Prominent Republicans, including party chairman Reince Priebus, have treated the businessman with kid gloves even as his rhetoric has inched further toward the fringes. Trump has previously discussed mounting a run as an independent if he is not treated “fairly” by party bosses, stoking GOP fears that he might eventually peel off voters from the party’s eventual nominee.

“I will support the nominee of the party,” said Arizona Sen. John McCain, the GOP’s 2008 presidential nominee. “I doubt if there’s any nominee I totally agree with in my lifetime.”

Pressed on how he could disagree vehemently with Trump on this issue, yet theoretically vote for him, McCain deferred to the limits of the two-party system. “I am a loyal Republican, and I rely on the good judgment of Republican voters.”

Some Republicans stated that the question was moot because the businessman would not win the GOP nod.

“He won’t be the nominee,” said Sen. Jeff Flake (Ariz.). “I don’t think he’ll win Iowa, and I don’t think he’ll play second fiddle to anyone. As soon as someone eclipses him, and he figures ‘I can’t say anything crazier than I’ve said to change the equation,’ then he’ll find a way to back out.”

Flake called Trump’s proposal “lunacy” and “just awful, frankly.”

“Just when you think he can’t stoop any lower, he manages to do so,” Flake said, wondering how Trump’s plan would affect diplomats and foreign officials who are Muslim from taking official visits to the U.S. He pointed specifically to a visit scheduled next month from Jordanian King Abdullah.

“I’m not sure he’d be able to come under a Trump presidency,” Flake said.

New York magazine's Ed Kilgore looks at why Republicans are afraid to say they would not support Trump: They're afraid he'll launch an independent run in the general election that would drain off the voters they need to win the White House:

Asked if he would renege on his earlier pledge to support the Republican nominee even if his name is Donald Trump, (Lindsey) Graham would not comment.

He knows — and I'm guessing he's been reminded repeatedly during the last few days — that if his rivals back out on that pledge, so can Trump. And an eminently viable independent run by an aggrieved and vengeful Trump could produce a catastrophe for the GOP next November.

Other presidential candidates and party leaders weighed in on Trump's Islamophobic excesses with varying degrees of heat, ranging from Jeb Bush's argument that it plays right into ISIS's hands to Ted Cruz's mild rebuke of the proposal yoked to a reaffirmation of his fond feelings toward the Donald. But nobody's tearing up the pledge and saying they won't back Trump if he raises his hands in triumph in Cleveland next summer.

Anyone forgetting the havoc Trump could create as an independent general-election candidate got a reminder today from a new national Suffolk/USA Today poll that not only shows him holding on to a robust ten-point lead over the GOP field, but also finds that 68 percent of his GOP supporters would follow him right out of the party if he ran as an indie. Trump himself gloatingly advertised this finding on social media. 

Talking Points Memo's Lauren Fox suggests "the establishment" couldn't stop Trump even if they wanted to:

There is a myth circulating in the beltway. The myth is that bombastic business mogul Donald Trump can be taken down if only a Republican donor were to write a big enough check, if only a fellow GOP contender were to attack him with a vengeance or if only someone, somewhere concocted the right campaign-combustion cocktail.

Pundits, pollsters, reporters and donors have been racking their brains in war rooms and newsrooms from Washington to Iowa since the summer trying to understand the best way to explain and then to solve the problem of Trump. Underlying most of the scenarios gaming out the GOP primary has been the universal assumption that, if they really wanted to, the GOP "establishment" could step in and put a stop to Trump.

It's not to say that Trump is going to win the nomination. There is plenty more action ahead, candidates to drop out and Trump stumbles to be had, but an organized campaign by the establishment isn't likely to be his undoing.