DES MOINES — One of the most enduring and unlikely story lines of this presidential campaign has been the passion that the oldest candidate in the Democratic race inspires in the youngest voters.

Sen. Bernie Sanders (I-Vt.) — who will be just eight months shy of his 80th birthday on Inauguration Day 2021 — has surged in no small part because of the support he is receiving from people under the age of 35, with whom he has a sizable double-digit lead in some polls. They were the core of his support four years ago, when he came in second to Hillary Clinton for the Democratic nomination, and remain so now.

But consider a less-discussed flip side to that: Sanders is struggling to win over Democrats closer to his own age.

In a national Quinnipiac University survey released this week, Sanders received only 7 percent support from Democrats and Democratic-leading independents 65 years and older; by comparison, former vice president Joe Biden was favored by 36 percent of those older voters. Biden enjoyed a 25-point lead over Sanders among those between 50 and 64 years old.

There are a number of possible reasons this is true, starting with the fact that older generations are more conservative than younger ones. Another is that the “democratic socialist” label that Sanders embraces means different things to different generations.

Younger people are more likely to associate it with progressive, Scandinavian-style systems that offer their citizens heavily subsidized health care, college and other generous government programs; for those who grew up during the Cold War, the word evokes memories of the Soviet Union and its failed command-and-control economy.

Nor are older people as enamored as younger ones with the specifics of the policies that Sanders supports.

A Quinnipiac poll in November found that nearly three-fourths of Democrats under 35 support Sanders’s Medicare-for-all proposal, but among all respondents actually eligible for Medicare, less than one-third do. That, no doubt, reflects a concern that putting millions more Americans into the program would threaten its stability, and with it, the value of the benefits provided by a system to which older Americans spent decades contributing.

None of this is to say that Democratic baby boomers and their elders don’t recognize that the health-care system needs reform. Older voters were more inclined to support a gradualist approach — dismissed by Sanders but backed by the more centrist candidates in the race — in which adults would be given an option of buying into Medicare if they choose.

All of this suggests that a potential undertow exists within Sanders’s recent burst of momentum in the Democratic primary.

Older voters have traditionally shown a higher propensity than younger ones to actually turn up at the polls. That means, for Sanders to pull out a convincing victory in Monday’s Iowa caucuses, he will need a higher-than-average turnout of new or infrequent young voters in a state where the median age of registered Democrats is in the mid-50s.

What this means for Sanders’s chances in a matchup against President Trump, however, is far less clear. The generational tensions within the Democratic Party are real, but a different dynamic will come into play as the race moves from the primary to the general election.

The latest Post-ABC News poll, which was conducted between Jan. 20 and Jan. 23, posed hypothetical matchups between Trump and each of the most talked-about Democratic contenders.

All of the Democrats ran stronger against Trump among young voters than they did among older ones, with Sanders doing only marginally better with 18- to 39-year-olds than Biden and Sen. Elizabeth Warren (D-Mass.). He got 55 percent, against Trump’s 41 percent; both Biden and Warren received 53 percent.

At the same time, the poll indicated that if Democrats chose Sanders as their nominee, he would not represent a significantly bigger drag with elderly voters than picking one of their other choices. Among voters 65 and older, Sanders got 47 percent to Trump’s 50 percent, which was within the poll’s margin of error. Biden got 48 percent; Warren did worse than both of them among that group, garnering only 45 percent.

Last fall, Sanders acknowledged that he has “a lot of work to do” to win over Democratic voters in his own age group. “I’m not saying we’re going to win older people by a huge number, but we’re going to do a lot better than we are right now,” he added.

His vow will be field-tested on Monday night with Iowa’s legions of graying caucus-goers, who may provide the first indications of whether the recent Sanders surge itself will have any longevity.

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