This raises the specter of a serious threat, should Warren or Sanders emerge as the nominee. You can call it the “your own man says so!” rule, named after schoolyard ballgame disputes, where the acknowledgment by a member of one team that his or her teammate was out settles the argument. It’s what happened when almost half the Republican Party refused to support Barry Goldwater because of his “extremist” views, giving his opponents an easy way to torpedo his candidacy. It happened when George McGovern’s opponents in his own party, including ex-Vice President Hubert Humphrey, accused him of ideas that would cut the muscle out of American defenses.

And a year from now—an eternity in campaign time, but not too long to keep the issue warm in a big oppo file—it wouldn’t be hard at all for Donald Trump, on Twitter and in ads and on a debate stage, to point out that a member of Warren’s own party, sharing the same stage, implied that her health care ideas would be dangerous for America. (The same applies in spades for self-identified socialist Sanders.)