Three days ago this newspaper ran a front-page story in which Muslim leaders called on their brothers and sisters in faith to condemn any radical extremism within their ranks. The Republican Party could really use its own “Not in our name!” campaign to marginalize Donald Trump.

Trump isn’t a real Republican, of course. He isn’t even a real conservative. The lifelong showman is playing a role.

And while his increasingly off-the-wall performance art doesn’t represent the majority view of Republicans, it is resonating with many angry and fearful voters, giving him greater standing than he deserves and damaging the GOP along the way.

But this is about so much more than political self-interest for one party.

Trump’s dark suggestion this week — that the United States should forbid all Muslims from stepping foot on these shores — is a breathtaking assault on this nation’s founding principles. It is constitutionally indefensible, not to mention wildly impractical.

The mere suggestion of a no-Muslim-entry policy reinforces the belief among many followers of Islam, both here and abroad, that the U.S. is at war not with radicalism — but with the faith itself. Ah, but it scored Trump a standing ovation in South Carolina and he continues to dominate the cable news shows. So what the hell does he care?

Some leading Republicans are speaking out. There are Trump’s rivals for the GOP nomination (with some mealy-mouthed exceptions). The Republican state party chair in New Hampshire, where the first ballots of the 2016 campaign will be cast in eight weeks, has abandoned her pledge of neutrality and called Trump’s comments what they are — “unconstitutional,” “un-American,” and “un-Republican.”

And House Speaker Paul Ryan stood before the cameras yesterday and declared Trump’s proposal “not what this party stands for and, more importantly … not what this country stands for.”

“I told our members this morning to always strive to live up to our highest ideals,” Ryan said, “to uphold those principles in the Constitution on which we swear every two years that we will defend.”

Trump has never been called upon to swear such an oath. He has no clue what doing so requires, and he should not be given the opportunity to find out.