As for the principle that elected representatives of the American people should be able to travel where they like — the principle enunciated a few weeks ago by Ron Dermer, Israel’s ambassador in Washington — Mr. Trump has made his rejoinder clear: Principles are for suckers. How many dictators are rejoicing today that a U.S. president has given them full permission to bar members of Congress who in the future might want to visit their countries to monitor elections or speak up for human rights?

By enlisting a foreign power to take action against two American citizens, let alone elected members of Congress, Mr. Trump crossed a line that other presidents have not, in effect exporting his partisan battles beyond the country’s borders. And he demonstrated the lengths that he will go to to target his domestic opponents, in this case two of the congresswomen of color he has sought to make the face of the Democratic Party heading into his re-election campaign.