Until his dying day, my dad’s Uncle Bern was a communist sympathizer. I remember him as an affable old man with a gracious wife who made a modest living selling antique lace. He probably wouldn’t have hurt a fly. Yet he found much to admire in the most murderous ideology of the 20th century, responsible for tens of millions of deaths from the killing fields of Cambodia to the gulags of Murmansk.

If you’re Jewish in America, chances are there’s at least one Uncle Bern somewhere in your family tree. As the scholar Ruth Wisse noted last year in Tablet magazine, Jewish intellectual life in the 1930s and 40s was largely defined by one’s stance toward one thing: The Party. Historians reckon that Jews accounted for nearly half the Communist Party’s total membership in those years, while many other Jews were close fellow travelers.

Most of these people, like my great-uncle, were deeply misguided idealists who otherwise led quiet and decent lives. A tiny handful of others — including atomic spies Julius Rosenberg, David Greenglass, Harry Gold and Morton Sobell — betrayed America’s most important military secrets to Stalinist Russia and did incalculable damage to the country and the world.

Here’s a thought experiment: Would the United States have been better off if it had banned Jewish immigration sometime in the late 19th century, so that the immigrant parents of Rosenberg and Sobell had never set foot here? The question is worth asking, because so many of the same arguments made against African, Latin-American and Muslim immigrants today might have easily been applied to Jews just over a century ago.