Mr. Ryan denounced Mr. Trump’s attack on an Indiana-born judge of Mexican heritage as “the textbook definition of a racist comment.” He added: “It’s absolutely unacceptable. But do I believe that Hillary Clinton is the answer? No, I do not.” Thus goes his justification for abandoning principles and good sense: The bizarre excesses of Mr. Trump — racism, xenophobia, paleo-protectionism — are nothing compared with the prospect of Mrs. Clinton winning the White House.

Unlike Mr. Ryan, other Republican leaders have backed away from Mr. Trump as a candidate gone amok, notably Mitt Romney, the presidential candidate who campaigned alongside Mr. Ryan. Even John Kasich, the convention’s host governor, has felt obliged to boycott the celebration of Mr. Trump. In contrast, Mr. Ryan seems to be trying to look just loyal enough to avoid blame for a Clinton victory while positioning himself to pick up the pieces of the party if Mr. Trump loses. But he cannot have this both ways. He is tying his future to Mr. Trump’s ugly campaign.

Even before the credibility challenge presented by the Trump candidacy, Mr. Ryan’s reputation as a congressional thinker and innovator was subject to increasing doubts, as was his leadership of fractious House Republicans in their failure to agree on basic budget resolutions. Their election year agenda, called “A Better Way,” has been widely criticized for lacking detail in its claims about curtailing poverty. Last month, after years of vowing to come up with a replacement for the Affordable Care Act, Mr. Ryan’s caucus finally offered a proposal. Far from being innovative, the plan was built around Mr. Ryan’s longstanding proposals to shift Medicare to a market-based competitor of health care companies.

Under scrutiny, Mr. Ryan’s prescriptions for reforms and savings have proved to be glossy variations on the tired Republican tactic of more tax cuts favoring the rich over the middle class, while his budget-cutting proposals have never added up to his grand promises.

As a conservative policy leader, Mr. Ryan should have every reason to oppose Mr. Trump. They hold different positions on core issues, including on immigration (Mr. Ryan opposes Mr. Trump’s call for 11 million deportations), trade deals (Mr. Ryan supports the Pacific trade deal that Mr. Trump has vilified) and international engagement (the speaker favors a stronger NATO and wariness toward Russia, while Mr. Trump seems ready to embrace isolationism).