Hillary Clinton’s memoir of her presidential campaign is getting most of the media attention this week, but that’s the politics of progressive nostalgia. If you want to know where the Democratic Party is going, Bernie Sanders showed the way Wednesday with his proposal for a complete government takeover of health care.

“Medicare for all,” the Vermont Socialist calls it, and what was once a crank idea is fast becoming a progressive litmus test for Democratic candidates. Fifteen Democratic Senators endorsed it, including possible...