Speaker Nancy Pelosi has directed committees investigating President Trump to “proceed under that umbrella of impeachment inquiry,” but the House has never authorized such an inquiry. Democrats have been seeking to impeach Mr. Trump since the party took control of the House, though it isn’t clear for what offense. Lawmakers and commentators have suggested various possibilities, but none amount to an impeachable offense. The effort is akin to a constitutionally proscribed bill of attainder—a legislative effort to punish a disfavored person. The Senate should treat it accordingly.

The...