Over the 18 months of the campaign, she got a close-up view of his unpredictable style and bombastic personality. “I think I fundamentally understand the way Trump thinks,” she said. “There were moments when he surprised me, but I was never shocked. I was not shocked when he won.”

Mr. Trump’s sudden rise mirrors that of Ms. Tur’s. Just two years ago she was a foreign correspondent for NBC, living in London. But she happened to be in New York when the future president announced his candidacy, on June 16, 2015.

“How would you like to spend the summer in New York?” an NBC News executive asked her. “We want you on Trump’s campaign. It will be six weeks, tops. But hey, if he wins, you’ll go to the White House.”

A year or so later, she had captured national attention. Colleagues rallied around her, thousands tweeted #iamwithtur, magazines came calling, and HarperCollins engaged her to write a book on the 2016 campaign; “Unbelievable” is due out in September. Then there is the afternoon anchor slot, which MSNBC gave her in January. In April, she received a Walter Cronkite Award for excellence. And this month she will become a contributor to the much publicized new NBC News program “Sunday Night With Megyn Kelly.”

But what she won’t do is go to Washington.

“I did not want to go to Washington for a number of reasons,” she said, “one of which, the first and foremost of which being I’ve got a personal life now, and I’m engaged. I want to be in New York with my fiancé. And I’m a big believer in reporting from the outside. I’ve always been an outsider. I think being in the White House press corps, it’s difficult to do the sort of journalism that I would want to do.”