Like most prominent Republicans at the time, Romney no doubt assumed that the fever swamp stuff didn’t need to be attacked, that it would evaporate once the G.O.P. won back the White House. But instead the fever swamp stuff helped hand the G.O.P. to Trump himself, and the birther’s grip-and-grin with an uncomfortable Romney was a small but notable milestone on that path.

The larger, indirect role that Romney played in Trump’s ascent was in the way he ran and lost in 2012. There were times when the Man From Bain Capital seemed to have some sense of the populist discontents that Trump successfully played upon four years later. Romney’s rhetoric on China and immigration was a more restrained version of Trump’s nationalist pitch, and here and there he tried to imitate Franklin Roosevelt’s promise, updated crudely by Trump, to be a traitor to his successful class.

But not nearly often enough. Instead, the defining pitch of the Romney campaign was the tone-deaf “you built that,” which valorized entrepreneurs and ignored ordinary workers; the defining policy blueprint was a tax reform proposal that offered little or nothing to the middle class; and the defining gaffe was the famous “47 percent” line, in which Romney succumbed, before an audience of Richie Riches, to the Ayn Randian temptation to write off struggling Americans as losers.

As a result, whether in his father’s Michigan, in his running mate’s Wisconsin, or in Pennsylvania where he campaigned hopefully near the end, downscale white voters who could have gone Republican either voted for Obama or stayed home. And in that failure lay the opportunity that Trump intuited — for a Republican candidate who would rhetorically reject and even run against the kind of corporation-first conservatism that Romney seemed to embody and embrace.