When a lackluster Scott Walker walked off stage after the Republican primary debate last week, his supporters had one question: What was he thinking?

The Wisconsin governor gave one answer on Monday, when he announced he was ending his campaign for the Republican presidential nomination.

“As a kid, I was drawn to Ronald Reagan because he was a Republican and a conservative. Most of all, I admired him because of his eternal optimism in the American people,” Walker told his supporters at a hotel in Madison, Wisconsin. “That thought came into my head when we were all standing on the stage at the Reagan Library last Wednesday.”

That debate, Walker said, had “drifted into personal attacks,” and away from “the basics of our party.”

“I believe that I'm being called to lead by helping to clear the field in this race so that a positive conservative message can rise to the top of the field,” he said. “With this in mind, I will suspend my campaign immediately.”

News of Walker’s exit was first reported by The New York Times.

Walker appeared to be the perfect candidate when he announced his run in mid-July. He had a strong record of winning elections, he was polling well in Iowa, impressing conservative activists, and wooing powerful donors. Then came the dog days of summer and Donald Trump, whose “caricature of a strong-willed populist conservative overwhelmed Walker’s chances at effectively running as a real strong-willed populist conservative,” as my colleague David Graham puts it. Other outsiders, like neurosurgeon Ben Carson, pushed Walker further into the background. Walker did some of that pushing himself, thanks to a handful of flip-flops and gaffes, particularly on foreign policy. One example: his comparison of Islamic State militants to union protesters in Wisconsin.