After decisive victories by Donald Trump and Hillary Clinton in the New York primaries, Democratic leaders need to send a strong, stern message to Bernie Sanders: Back off. Stop attacking Hillary. If you want to stay in the race to propound your policy ideas, fine. But don't keep undercutting the one candidate who can save the party -- and the country -- from a President Trump.

Lately, Sanders has been going in the opposite direction. As his chances for the Democratic nomination slip away, he's turned angry and acerbic, providing Republicans with an arsenal of ammunition to use against Clinton in the fall. That's a dangerous and selfish game.

Some Republicans keep insisting they can stop Trump on their own. Clearly he fails to command a majority in his own party, with only 2 out of 5 Republicans backing him in national polls. Among all voters, his favorable rating in the latest Wall Street Journal/NBC poll stands at 24 percent, with his negatives soaring to 65 percent -- the worst showing in the history of that poll for a major presidential contender.

But suppose Trump falls short of a majority. As Robert Gates, the former defense secretary, is fond of saying: The three words not spoken often enough in Washington are, "and then what?"