Republican presidential nominee Donald Trump speaks as Democratic nominee Hillary Clinton listens during the second presidential debate. Saul Loeb/AP At a rally on Saturday, Donald Trump suggested — with no evidence — that Hillary Clinton may have been on drugs at the second debate, and he proposed that drug tests should be required before the next debate.

"I'm willing to do it," he said of mandatory pre-debate drug testing.

Clinton should take him up on the offer.

You may remember former Vermont Gov. Howard Dean got in some trouble for suggesting Trump might have a cocaine problem after he sniffled his way through the first debate. Not unreasonably, Trump's supporters complained that this was a baseless accusation.

But now, Trump himself has chosen to put the "Who acts like they are on drugs?" issue in play. And — like "Who's gropiest?" — it's a question Trump shouldn't want the campaign to focus on.

Clinton, after all, is not the candidate in this race whose mood and energy level are wildly erratic. She is not the candidate who goes on 3 a.m. tweetstorms against former beauty queens. She is not the candidate whose accusations against her opponents tend to be projections of her own problems. And she is not the candidate with a case of the sniffles.

I am not saying Trump has a drug problem. He probably just has a defective personality that causes him to behave as though he has a drug problem.

But, as Trump notes, the only way to tell for sure is if the candidates take drug tests.