The press spends a lot of energy tracking the many errors in Donald Trump’s loose talk, and during Monday’s presidential debate Hillary Clinton expressed hope that fact checkers were “turning up the volume” on her rival. But when it comes to the Federal Reserve, Mr. Trump isn’t all wrong.

In a looping debate rant, Mr. Trump argued that an increasingly “political” Fed is holding interest rates low to help Democrats in November, driving up a “big, fat, ugly bubble” that will pop when the central bank raises rates. This riff...