John McCain has laid it on the line to the president of the United States.

If you’re going to make an explosive allegation against your predecessor, Mr. President, it is imperative that you tell Americans the “basis on which” you are making that allegation.

That’s what McCain has told Donald J. Trump to do.

Trump ignited a firestorm over the weekend when he rolled out of the sack at 6 in the morning and fired off a tweet that said President Barack Obama “ordered” a wiretap of Trump’s offices in New York.

No proof. No evidence. No attribution. Nothing accompanied the tweet. But the flames began burning out of control.

McCain says a simple tweet isn’t good enough.

To my ears, it sounds as though the Arizona Republican — and 2008 GOP presidential nominee — doesn’t exactly believe what Trump has asserted.

At issue, of course, is the reported relationship between Trump’s campaign and Russian government officials. Trump asserts that Obama had the phones bugged so he could eavesdrop on Trump’s campaign officials to learn whether there was a relationship with a foreign government during the 2016 presidential campaign.

Trump has accused Obama of breaking the law. He has essentially called his predecessor a felon. Presidents cannot order phones to be bugged. These things occur through a warrant issued by a federal judge at the behest of the Justice Department.

Sen. McCain is insisting that Donald Trump tell us the basis of his wild-ass allegation.

Well, Mr. President? What is it? Talk to us. You are the president of the United States of America. You owe us — your bosses — a complete and thorough explanation.