We agree with Donald Trump that he has no moral obligation to defend the President every time somebody says something bad or controversial about him. Which is what Mr. Trump tweeted in respect of his decision to stand silent when asked at one of his political events about the talk that President Obama is a Muslim. “Am I morally obligated to defend the president every time somebody says something bad or controversial about him?” The Donald demurred. Fair enough, we say.

In the unlikely event that he wins the presidency, though, Mr. Trump would have a brightline moral obligation. And one that couldn’t be more relevant at this juncture. This would be the obligation to defend the Constitution, to which every president is sworn. That Constitution lays down a prohibition against religious tests for public office. Had Mr. Trump marked that point when a quesitoner started bellyaching about Mr. Obama’s religion, the candidate would have signaled he understands the parchment to which he’d have to be bound.

The right thing to say is not simply to deny that Mr. Obama is a Muslim. It’s not merely to point out that Mr. Obama is a Christian. The right thing is to say that under our Constitution, it can’t matter. The right thing to say is that no religious tests shall ever be required for the presidency or any office or public trust under the United States. It is right there in the main body of the Constitution, in Article 6. It is, these columns have often stressed, the most emphatic statement in the entire American parchment.

It’s not just Mr. Trump who’s in default. Why doesn’t the Left stand on this constitutional bedrock in defending Mr. Obama from the religious rumors? That is the most sensitive point. The Left doesn’t want to use the religious test clause to defend Mr. Obama because it wants to use religious tests to enforce its secularist campaign. This is evident in the battle over the clerk of Rowan County, Kentucky, who avers that her religion prevents her from licensing same-sex marriages.

The clerk is arguing right now in federal court that requiring her to issue such licenses under her own name is a religious test that would prevent Apostolic Christians such as herself from ever serving as clerks at Rowan County. She might be right, she might be wrong. But the Left wants that claim dismissed out of hand, without even a full due process in court, and the judge seems to be of a similar mind. Wouldn’t it be nice were at least one candidate to signal that he or she grasps the meaning of this clause to which all officers of America are morally obligated?