What if Lesley Stahl had stopped Donald Trump right in his tracks?

What if she had simply dug in her heels and refused to budge when Trump, the presumptive Republican presidential nominee, insisted — once again, this time on “60 Minutes” on Sunday evening — that he had opposed the war in Iraq from the beginning?

Because that claim, which Trump has made a cornerstone of his campaign, is “blatantly false,” according to The Washington Post’s Fact Checker and many other similar efforts. Politifact, the Pulitzer Prize-winning fact-checking operation, also called it false. And BuzzFeed dug up a 2002 interview in which Trump said he supported the invasion.

As the Fact Checker’s Michelle Ye Hee Lee wrote earlier this year in a piece on eight falsehoods Trump repeated in a 16-hour period:

“Trump did not oppose the Iraq War before 2004, as we and countless other media outlets have found. We compiled a complete timeline of all his public statements in 2002 and 2003 relating to the Iraq invasion and found no evidence to support this. . . . We checked with a dozen former Bush White House officials, and none could recall a meeting with Trump, concerns about his opposition, or even Trump’s views being on their radar prior to 2004. We awarded this claim Four Pinocchios.”

But Stahl — busy trying to herd the other rhetorical cats set loose in the interview — did not say what she should have, something like this: “No, Mr. Trump, that is simply false, and I’m not going to let that go unchallenged.”

Instead, she let the man who could be president get away with it, basically affirming his falsehood by twice saying, “Yeah,” as he stated it.

Then, Stahl ended that segment of the interview (which had to do with his vice-presidential pick, Indiana Gov. Mike Pence, and presumptive Democratic presidential nominee Hillary Clinton, who both voted for the invasion) with these agreeable words: “Got it.”

DONALD TRUMP: But I was against the war in Iraq from the beginning.

LESLEY STAHL: Yeah, but you’ve used that vote of Hillary’s that was the same as Governor Pence . . . as the example of her bad judgment.

TRUMP: Many people have, and frankly, I’m one of the few that was right on Iraq.

STAHL: Yeah, but what about he —?

TRUMP: He’s entitled to make a mistake every once in a (laugh) while.

STAHL: But she’s not? Okay, come on —

TRUMP: But she’s not —

STAHL: She’s not?

TRUMP: No. She’s not.

STAHL: Got it.

I’ve asked “60 Minutes” for comment on why Stahl did not challenge Trump’s obvious falsehood and in fact seemed to agree with it. But here’s my take: Trump says whatever comes into his head that serves him best in the moment. He’s amazingly skilled at it.

But, slippery as he is, this is too important to let slide. As journalists continue reporting on the Great Dissembler, they need to be just as skilled and persistent at what they do. They should not let Trump, or any candidate, get away with lying to citizens.

For more by Margaret Sullivan visit wapo.st/sullivan