Speaker Paul D. Ryan was in a hotel room in Cincinnati last May when he learned that Donald J. Trump — a man he barely knew, with no institutional ties to his party and a mouth that had already clacked his nerves — had secured the Republican nomination for president.

Mr. Trump had recently lost the Wisconsin primary, which Mr. Ryan and other Republicans in the state thought would presage the reality television star's demise. Senator Ted Cruz of Texas had made strides, and Mr. Ryan thought the campaign would snake into the summer.

But he badly miscalculated. The ensuing path, from Mr. Trump's securing the nomination to Mr. Ryan's decision not to have the nominee appear with him at a rally in Wisconsin a month before Election Day, has been jagged and treacherous, marked by brief moments of hope that Mr. Trump could be controlled, followed by the sinking realization that he could not.