Democrats are determined to remove President Trump from office in 2020—or before, if that becomes possible. If precedent holds, he can be unseated in 2020 by a candidate perceived as his opposite: experienced, serious, knowledgeable about policy. If Democrats attempt to rush the process, amid current charges that Mr. Trump is a “traitor” and Russian agent and that his Supreme Court nominee is an extremist, they will further energize their take-it-to-the-streets wing but alienate all but partisan Northeast and Pacific Coast voters.

The...