House Speaker Paul D. Ryan (R-Wis.) in Washington on Wednesday. (Win Mcnamee/Getty Images)

DECLARING THAT “we shouldn’t accept ugliness as the norm,” House Speaker Paul D. Ryan (R-Wis.) delivered a fine speech Wednesday on the tenor of the nation’s politics, calling on political leaders as well as ordinary Americans to aspire to a more reasonable debate. Unfortunately, Mr. Ryan did not directly confront the chief threat to that goal: GOP front-runner Donald Trump. Mr. Ryan’s lofty oratory alone will not clean him or his party of Mr. Trump’s moral tarnish.

On their own, Mr. Ryan’s words were quite commendable. He argued that the business of political leaders was to propose policies and put the quality of their ideas to the test. He warned against “playing to your anxieties” or “playing the identity politics of ‘our base’ and ‘their base.’ ” He said that politicians should not “resort to scaring you,” and that Americans should not presume that those who disagree with them do so in bad faith. He also admitted that he was wrong to write off a large piece of the country as “takers” in the 2012 presidential election.

Unfortunately, the nation’s political discourse has deteriorated considerably since 2012. There may be many reasons for this, including large structural and demographic forces to which politicians respond but over which they have limited control. Yet there is little doubt that Mr. Trump has not only exploited emerging divisions but also enthusiastically pried them open.

While Mr. Ryan warns against playing identity politics and appealing to people’s anxieties, Mr. Trump this week continued to exploit fear of Muslims and illegal immigrants, calling to “close up our borders,” warning that immigrants “could be ISIS” and arguing, with scant evidence, that the Belgium terrorist attack might have been prevented if Western authorities had tortured a captured extremist.

While Mr. Ryan calls for focusing on substance, Mr. Trump has run a uniquely substanceless campaign, telling us Monday that his solution to inner-city poverty was “giving them spirit.” While Mr. Ryan attacks gutter politics, Mr. Trump defended his comments on the size of his hands and other organs by saying he “had to do it” because Sen. Marco Rubio (R-Fla.) “started it.”

Given the gaping divergence between Mr. Ryan and Mr. Trump on essential civic values, it is a wonder that these two men remain in the same party. It is more than a wonder — it is a shame — that Mr. Ryan has not repudiated Mr. Trump and promised to oppose him. Doing so would have made clear that Mr. Ryan puts the principles he advanced before partisan loyalty and political calculation, where they firmly belong. Instead, Mr. Ryan continues to be another not-so-innocent bystander as his party slides toward Trumpism.