Here is a transcript of the exchange:

BARBARO: Does he have the mental fitness, the kind of psychological suitability to the office of the presidency? GINGRICH: [pause] Yeah, and my answer would be, sure. BARBARO: Sure? GINGRICH: Sure. I mean, he is at least as reliable as Andrew Jackson, who was one of the most decisive presidents in American history. Nobody would have predicted Abraham Lincoln’s capabilities before he became president, and most people didn’t believe him while he was president. BARBARO: Can you be more forceful than “sure”? GINGRICH: [pause] I think that Trump has a willingness to break up a system which is decaying. I think the kind of personality that is prepared to be outside the total establishment and have the self-confidence to take on the establishment of both parties is a personality which will by definition not be normal. It will not be a good, corporate managerial, go-along-and-get-along kind of guy, and that’s what you guys mean by temperament.

(Andrew Jackson was last in the news when the Treasury Department decided in April that the former president, a slaveholder known today for both his persecution of Native Americans and advocacy for poor whites, would share the $20 bill with the former slave and abolitionist Harriet Tubman.)

Questions have mounted among both Democrats and Republicans about whether Mr. Trump possesses the mental fitness and strength of character to serve as president, and his poll numbers have fallen as a result.

Mr. Gingrich acknowledged Mr. Trump’s trouble in his interview with “The Run-Up.” “The last two weeks have been peculiarly bad for Trump,” he said.

Mr. Trump has appeared to inflict much of that damage upon himself, including by engaging in a days-long feud with the parents of a fallen United States soldier who criticized him at the Democratic National Convention, confounding many of his supporters.