As recently as March, Wisconsin Gov. Scott Walker (R) was the nation’s #1 Republican presidential contender. As recently as May, the far-right governor wasn’t just leading in Iowa, he was also the only candidate with double-digit support.

A month later, in June, Walker kicked off his national campaign to significant fanfare, and it was easy to see him as a top-tier contender for the GOP nomination – Walker offered a unique combination of establishment credibility, right-wing appeal, and electoral success. There was ample speculation that the fight might very well come down to Walker and Jeb Bush.

That was in June. Today, Walker’s done . NBC News’ Chuck Todd confirms:

Wisconsin Gov. Scott Walker is set to drop out of the presidential race Monday night, a source close to the campaign confirms to NBC News. He is scheduled to hold a press conference in Madison, Wisconsin, at 6 p.m. ET to announce his exit from the race.

I’ve seen some comparisons of late between Walker and former Minnesota Gov. Tim Pawlenty (R), who also briefly ran in 2012 before watching his support evaporate.

But the similarities are limited. Yes, they’re two bland, Midwestern governors, but Pawlenty never really developed a strong base of support and few ever saw him as a frontrunner.

Walker, on the other hand, was widely seen as a powerhouse, who had put all of the pieces in place to compete over the long haul. It makes the Wisconsinite’s three-month flameout that much more extraordinary – few presidential candidates ever make the transition from hero to zero this quickly.

So what in the world happened?

1. Donald Trump happened. Walker saw himself as the principal rival and credible alternative to Jeb Bush and the party establishment, but over the summer, those looking for an anti-establishment alternative turned instead to a reality-show host and retired right-wing neurosurgeon.

2. The wrong resume at the wrong time. Walker brought an impressive c.v. to the Republican base: a career in public service, a record of electoral successes in a “blue” state, and a series of far-right accomplishments. On paper, that sounded like a recipe for success. In practice, the same GOP voters Walker reached out to were the same voters who decided they preferred someone in the White House who has no background whatsoever in elected office.

3. Walker was a genuinely awful campaigner. Perhaps the first sign of trouble was when Walker said union-busting in Wisconsin prepared him for national security challenges against ISIS. There was also all the borderline-creepy rhetoric about Reagan, whose record Walker didn’t seem to understand at all. For months, there was . Perhaps the first sign of trouble was when Walker said union-busting in Wisconsin prepared him for national security challenges against ISIS. There was also all the borderline-creepy rhetoric about Reagan, whose record Walker didn’t seem to understand at all. For months, there was one incident after another in which the governor seemed wholly unprepared for the rigors of a national campaign. It was often hard to watch.

When a candidate enters freefall the way Walker did, it’s usually the result of a scandal, a humiliating revelation, or a disastrous public humiliation. In the case of the Wisconsin governor, however, the truth is even more straightforward. When Republican voters looked for excitement, Walker was uninspiring. When the GOP base asked for a fresh face, Walker offered his background as a career politician. When voters looked for populism, Walker offered, well, the exact opposite of populism

The governor was the wrong man at the wrong time with the wrong message. Walker departing now saves himself the trouble of explaining more embarrassing defeats later.