The circus surrounding what in any other year would be a statement of the obvious — a newly installed speaker of the House does not wish to undo the will of thousands of voters and delegates in the dark of night at his party’s convention — is simply the latest strange turn in a race replete with them. And with each twist, it seemed, more Republicans not aligned with Mr. Trump or Mr. Cruz saw an elegant solution in Mr. Ryan. On Tuesday, he said he would have none of it.

“I simply believe that if you want to be the nominee — to be the president — you should actually run for it,” said Mr. Ryan, who will be the convention chairman. “I chose not to. Therefore, I should not be considered. Period. I just think it would be wrong to go any other way.”

By coming down so firmly against drafting a new candidate in Cleveland — the most coveted white knight denouncing the very idea — Mr. Ryan made it less likely that the party would risk the backlash from supporters of Mr. Trump and Mr. Cruz that would come from such a maneuver.

But if Mr. Ryan was emphatic that Republicans should not attempt to find a new candidate — or at least one who did not run for the presidency this election season — he also again made clear that he hoped to thwart many of Mr. Trump’s ideas, if stopping short of actually opposing him on a ballot.

“From his policies to his tone, I think Ryan is looking toward what the future of the party is going to look like, post-Donald Trump,” said Liesl Hickey, a veteran congressional aide and former executive director of the House Republican campaign committee.