This doesn’t sound Jewish, but it is. It’s an attitude born in Eastern Europe in the late 19th century and then brought to the slums of the Lower East Side, not long before Mr. Sanders’s father, Eli, arrived in this country in 1921 and settled in Brooklyn.

Jewish immigrants actively sloughed off the religion and traditional values of their parents and adopted socialism as a new kind of faith. By the 1930s, most of this community transferred its loyalty into devoted support for President Roosevelt and the New Deal, championing a liberal ethos that still characterizes American Jews today (about 70 percent of whom reliably vote Democratic in general elections).

A purer socialism also lived on in this community longer than it did anywhere else, with the Yiddish newspaper The Forward, a mainstay of support for America’s Socialist Party through the 1920s and into the ’30s. (The paper’s radio station, WEVD, was named after Eugene V. Debs, the perpetual Socialist candidate for president.) Left-wing intellectuals like Irving Howe carried on this thinking into midcentury, and Howe himself was flummoxed by the identity politics that emerged on the left in the late ’60s — a distraction, he believed, from the essential problems of capitalism.

For many, Mr. Sanders appears a throwback to this radical past, one that seems to leapfrog the generations. My Twitter feed during his victory speech in New Hampshire lit up with people saying that they felt like they were listening to someone railing from a soapbox on Delancey Street in 1928.

In other ways, though, even if he looks like a grandfather to his millennial supporters, he is actually representative of the direction the American Jewish community is headed. In those intervening generations, a majority of American Jews have tried hard to balance their liberalism with an identity that was also connected to tradition and religion, through Reform and Conservative Judaism, and an allegiance to Israel. But a 2013 survey by the Pew Research Center suggests that the socialist worldview is winning out. When asked what it means to be Jewish, 69 percent of respondents answered “leading an ethical life,” and 56 percent chose “working for justice and equality.” Only 19 percent said it had to do with “observing Jewish law.” Reflexive support for Israel has also declined.

These trends, which include increasing intermarriage, might eventually mean that the attempt to create a specifically American Jewish identity has largely been abandoned. This could be one more reason the Jewish establishment didn’t greet Mr. Sanders’s historic win in New Hampshire by hoisting him up in a chair like a joyous bar mitzvah boy. They see in him a reflection of these dismal statistics.

But will Mr. Sanders’s radical universalism help his candidacy?

As the primaries head toward states with larger black and Hispanic populations, his strategy against Mrs. Clinton’s longer relationships with those communities is to counter an appeal to group allegiance with his focus on income inequality. There are some signs that this is working: National polls show a majority of young women are rejecting calls from Clinton supporters to vote according to gender solidarity.