Now Mr. Krugman has become the kind of “pop internationalist” he once despised, and he leans against TPP. Try naming three prominent liberals or Democratic heavyweights who support the president on trade. Such is the state of a party in which Barack Obama is the extreme-right flank.

The Democratic turn against trade is part of the wider progressive march away from the centrist moorings that anchored it in the 1990s and made it a winner. But that march is also part of the liberal retreat from the world, and from the idea that the U.S. should serve as an economic, political and security anchor for free nations everywhere.

Does it matter to Democrats that TPP is also the economic centerpiece of the “pivot to Asia” that marked Hillary Clinton’s tenure at the State Department? Does it matter to them that China would like nothing more than to see TPP fail, so it can step in with its own version of a Greater East Asia Co-Prosperity Sphere? Does it even matter to Mrs. Clinton, or has she also moved on?

As with trade so with terrorism. The old liberal critique of the war in Iraq was that it was a distraction from the real threat to the U.S., al Qaeda and its jihadist affiliates above all.