CHIPPEWA FALLS, June 8 -- The breaking point was not the rain hammering the barn's metal roof, nor the technical glitches that interrupted the sitting twice. It was a single question Kristen Welker had asked - and re-asked - for four minutes: where is the evidence?

President Donald Trump ended his appearance on NBC's "Meet the Press" on Sunday by standing up mid-sentence, pulling off his lapel microphone, and walking off the set of a Wisconsin farm where the interview had been taped the previous Friday. "Let's call it quits because I've had enough," he told Welker before leaving. "Thank you, darling. Have a good time."

He crushed the mic underfoot on the way out.

What made the walkout notable was not the theatrics - Trump has a long hi...