William McGurn:

Presidential debates typically don’t determine the outcomes of elections, notwithstanding the large television audiences they draw and the dramatic moments they produce. But Tuesday night’s dustup between Donald Trump and Kamala Harris may be different.

Press failure has inflated it into the seminal event of the Trump-Harris race. Because reporters haven’t insisted that Ms. Harris answer basic questions, the debate, moderated by ABC News, may provide the only moment in the 2024 election when Americans get to see how Ms. Harris performs under pressure.

This failure would be appalling at any time, but the circumstances of Ms. Harris’s campaign turn simple media bias into journalistic malpractice. The vice president secured the top slot on the Democratic ticket without having to contest a single primary—and therefore without having to lay out and defend her record. This leaves her largely unknown to American voters, a situation Ms. Harris is now exploiting to reinvent herself as a moderate challenger rather than a woke incumbent.

In addition, Ms. Harris is a mother lode of unanswered questions on most of the issues that once defined her. This includes her previous support for everything from defunding the police and banning plastic straws to getting rid of Immigration and Customs Enforcement and starting from “scratch,” stances she now apparently disavows.