By the time the evidence is in during the trial of Mr. Trump, by the time the Republicans are reduced to muttering, spluttering defenders of the indefensible, there may be something approaching the two-thirds needed to convict. Even if the final count falls short, Mr. Trump and the Senate Republicans will have been so damaged that the Senate as well as the presidency will fall to the Democrats. Come January 2021, the nightmare that is Mr. Trump will be over. No political party whose president was embarrassed by impeachment proceedings has ever won the next election.