Political junkies are focused on Donald Trump, but Hillary Clinton’s weakness remains a huge story. Her support among Iowa’s likely Democratic caucusgoers has dropped to 37%, down 20 points since May, according to an Aug. 26 Des Moines Register/Bloomberg Politics poll. Meanwhile, Vermont Sen. Bernie Sanders is up 16 points to 30%, and Vice President Joe Biden, who isn’t even running, is up six points to 14%.

The poll caused great consternation among Mrs. Clinton’s supporters, as it should have. Some of her advocates tried to claim that the survey contained good news, in that the former secretary of state’s favorable rating among Democratic caucusgoers remained at 77%.

But if Mrs. Clinton’s favorables remained high while she hemorrhaged support, then Iowa Democrats are moving to Mr. Sanders not because they dislike her but because she doesn’t excite them, and they like him more. That too was polled. Only 2% of Mr. Sanders’s backers said that their decision to support him was because they don’t support Mrs. Clinton.

But is there a Democrat who—absent something extraordinary, say, Mrs. Clinton’s indictment over mishandling classified information on her private email account—can beat her for the nomination?

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Karl Rove joined Fox News Channel as a political contributor in February 2008. He also currently serves as a columnist for the Wall Street Journal. Mr. Rove helped organize the political-action committee American Crossroads. His latest book is "The Triumph of William McKinley: Why the Election of 1896 Still Matters" (Simon & Schuster, 2015). Follow him on Twitter @KarlRove.