Kamala Harris and Nancy Pelosi should have a talk. Both answered the siren song of the modern Democratic left. On Tuesday, Sen. Harris’s campaign broke up on the rocks. Meanwhile Speaker Pelosi, who stuffed her ears this summer against the left’s calls to impeach President Trump, sails on toward impeachment. That won’t end well either.

It is said endlessly that disagreement in politics is a good thing. Except when one side of the argument is occupied by America’s political left, which in its modern incarnation allows little...