In Cold War travels, Bernie Sanders found much to admire behind enemy lines. Now that’s a problem for his campaign.

The mayor of tiny Burlington, Vt., was back from Nicaragua and eager to share the good news.

The country’s Soviet-backed government — forged via armed rebellion — was cutting infant mortality, reducing illiteracy and redistributing land to peasant farmers. Its Sandinista leaders, branded terrorists by the U.S. government, impressed him with “their intelligence and their sincerity.”

Three years later, Bernie Sanders was fresh off the plane from Moscow, reveling in the beauty of the land and the contentedness of the people.

And a year after that, he returned from Cuba...