WASHINGTON – Sooner or later, the cool self-assurance of citizen non-politician Ben Carson was bound to betray him.

His categorical statement that “I would not advocate that we put a Muslim in charge of this nation, I absolutely would not agree with that,” was astoundingly un-American in itself.

Particularly as an African-American seeking the highest office, he seemed unaware or at least unconcerned that he was challenging the Constitution he so stoutly and repeatedly defends on other occasions.

Carson seemed to take issue with Article VI, which specifies that “no religious test shall ever be required as a qualification to any office.”

It was a brainless remark, astonishing from the famous retired Johns Hopkins neurosurgeon known for clear and uncluttered thinking and exposition.

For once, the credential of being a political outsider has tripped him up, as if he were the latest Sarah Palin out to lunch in terms of the nation’s founding document, crafted 228 years ago in Philadelphia.

Carson was inadvertently led into this unforgiveable gaffe by a similar lapse by fellow candidate Donald Trump. An anti-Muslim Trump supporter asked a question alluding to the conspiratorial canard popular in conservative circles that President Obama is a foreign-born Muslim. Trump, rather than setting his interrogator straight, let the accusation go uncorrected.

Carson, asked about the exchange, insisted he did not believe the religion of Islam was consistent with the American Constitution.

Up to now, the conspicuously even-tempered Carson has floated through the early stages of the 2016 Republican presidential campaign as a detached observer amid a pack of combative contenders. Of the three most prominent “outsiders” in the race – Trump, former Hewlett-Packard executive Carly Fiorina and himself – Carson has largely managed to appear above the bickering that has turned the competition into a catfight.

Wisconsin Gov. Scott Walker, in dropping out of the race, sought to separate himself from the brawling that some say has torn down the Republican Party.

He cited a ludicrous rationale that he was doing so to encourage others to bow out, too, “so that a positive conservative message” can get through, by somehow disposing of the disruptive Trump candidacy.

But Carson’s gratuitous slap at all American Muslims challenges his image as a level-headed non-politician of benign temperament and open mind, which seemed to distinguish him from the rabid partisans amid the large but now diminishing Republican field.

He is not likely in the weeks ahead to get quite the free ride from news media interrogation he has generally enjoyed up to now.

By and large, the “outsiders” have had a relatively easy course to navigate in this cycle’s GOP presidential competition. Carson and Fiorina have undergone little investigation of their occupational credentials so far.

Fiorina’s recent rise in the polls, however, has brought a deeper examination of her corporate career, which was cut short in 2005 when the Hewlett-Packard board forced her to resign amid a rout of the company’s share price.

As for Trump, thanks to his preening self-confidence and skill as a debating counterpuncher, he has survived an intensive press assault and an increasing pushback from some of his fellow candidates.

Former Florida Gov. Jeb Bush, a particular Trump target from the start as an early front-runner, has taken on the principal role of trying to cut him down to size.

Doing so has come at considerable cost in the polls to Bush, but he perseveres in the hope that the Trump fever will break sooner or later and allow him to regain his previous competitive edge.

The chances of the other more establishment candidates, including Florida Sen. Marco Rubio, Ohio Gov. John Kasich and New Jersey Gov. Chris Christie, also ride on the fall of the outsiders Trump, Carson and Fiorina, and a renewed confidence in experienced officeholders in the weeks ahead.

Editor’s note: Jules Witcover’s latest book is “The American Vice Presidency: From Irrelevance to Power,” published by Smithsonian Books. You can respond to this column at juleswitcover@comcast.net.