Rep. Adam Schiff, who prides himself on being one of President Trump’s leading Democratic tormentors, argued last week that the president must be impeached pronto so that he doesn’t “cheat in one more election.” I know Democrats are eager to expand the scope of impeachable behavior, but since when do we impeach presidents based on what they might do in the future?

Mr. Schiff ought to reacquaint himself with special counsel Robert Mueller’s testimony before Congress earlier this year. Mr. Mueller stated that his investigation...