Still, even if Trump bolts this year, it’s hard to see a lasting party emerging out of his candidacy. His voters represent, if anything, a less cohesive group than Roosevelt’s did in 1912. And just as no Progressive Party would have emerged that year without TR, so Trump’s voters almost certainly wouldn’t be coming together this year if it weren’t for Trump.

Nor, for that matter, would a Trump victory in Cleveland change the GOP as much as some are predicting. Too many of its stalwarts are deeply committed to policies of limited government and low taxes for the GOP to rewrite its basic script. The party may well soften its maximalist positions on a few issues to regain the allegiance of the middle-class whites flocking to Trump. But it will probably tweak, rather than overhaul, its stances on its core issues.

As the story of 1912 suggests, third parties usually need a compelling figure at their helm in order to gain traction. But the same reliance on personality can leave the new party weak after the leader exits the scene. By 1916, the Republican Party managed to regroup and mount a credible (if losing) general election campaign. And so the Progressive Party that Theodore Roosevelt started at the gates of Armageddon turned out to be, for all the wild excitement it generated, a footnote in our history.