LAS VEGAS — The smart money on Wednesday’s matchup between Hillary Clinton and Donald J. Trump was for a 12-round, no-holds-barred bout worthy of this city’s prizefights.

But Chris Wallace wanted a debate. And for the most part, he got one.

The first Fox News journalist chosen to moderate a general-election debate, Mr. Wallace mixed humor with scolding and persistence with patience to guide his charges toward the most substantive encounter of an unusually vicious election.

Mr. Wallace’s watchword for the evening, it seemed, was “defuse”: He posed detailed questions on policy, opened the proceedings with a calm query about Supreme Court jurisprudence and kept an eyebrow firmly arched as Mrs. Clinton, and more often, Mr. Trump, attempted to talk over him.