For the sake of argument, let’s stipulate that Donald Trump is everything Democrats say he is: a president who abuses his national-security powers by siccing a foreign government on his political rival, a racist/bigot/nativist constantly using “dog whistles” to stoke division, a man uniquely unfit to sit in the Oval Office.

Assume Mr. Trump is all these things. With an election scarcely a year away, the question then becomes: Why impeach him now? Surely a president as abominable as this ought to be easy to defeat at the polls....