The Democratic Party has a broad and diverse religious coalition — which, as Laura Bronner and I wrote last week, can be challenging for any candidate hoping to use religious rhetoric to rally voters. But what, might you ask, about the Democrats who aren’t religious?

People who identify as atheists, agnostics or “nothing in particular” accounted for 35 percent of Democratic primary voters in 2016, according to the Cooperative Congressional Election Study, and as we mentioned in our article, are a growing constituency within the Democratic Party. And according to crosstabs from Morning Consult’s weekly tracking poll for May 20-26, support for Sen. Bernie Sanders is higher among religiously unaffiliated voters than among religiously affiliated voters. Former Vice President Joe Biden, meanwhile, has an edge among religious Democrats.

Biden has a lead among most groups in the Democratic primary electorate, so Sanders’s relatively high levels of support among unaffiliated voters are worth probing. Yes, Sanders is a political rarity because of the openness with which he’s spoken about his lack of faith, but why are nonreligious voters more drawn to him? And could other Democratic candidates replicate his success? To better understand his appeal among the unaffiliated, I identified a couple of factors that could help explain it. None add up to a straightforward strategy for reaching these voters, since they’re disconnected from religious institutions and don’t have a clear set of shared political priorities. But they are worth watching because they make up a rapidly growing segment of the Democratic Party, and one that candidates might benefit from activating — if they can figure out how.

At this point in the primary, religiously unaffiliated Democrats seem more open to a candidate who isn’t Biden than their religious counterparts. And while there is some variation in the level of support for Biden, he’s the clear front-runner among several major religious groups. Among nonreligious Democrats, though, Biden is less of a clear favorite. Instead, a plurality (30 percent) of atheists prefer Sanders, while agnostics are split between Biden and Sanders. And although Biden is the favored candidate of 32 percent of Democrats who say they’re “nothing in particular,” nearly one-quarter support Sanders. Meanwhile, Sen. Elizabeth Warren is also somewhat more popular among atheists and agnostics than she is among other religious groups.

The most obvious explanation for the split among religious and nonreligious voters is that religiously unaffiliated Democrats — in particular, atheists and agnostics, who together accounted for 17 percent of primary voters in the 2016 CCES study — are substantially more liberal than Democrats who are still part of organized religion. And very liberal voters are a key constituency for Sanders. “Many religiously unaffiliated voters overlap with the liberal wing of the Democratic Party, so it’s not surprising you’d see higher support for Sanders,” said Ryan Burge, a political science professor at Eastern Illinois University. Religiously unaffiliated Democrats also tend to be younger overall — another group in which Sanders tends to be strong relative to Biden.

Although Sanders hasn’t really tried to hide his lack of religiosity, he also hasn’t used it to directly target unaffiliated voters. In interviews and debates, he has said he believes in God and is proud of his Jewish heritage, but he’s also said he’s “not actively involved with organized religion” and has mostly steered clear of the subject. So it’s possible that reaching these voters doesn’t require an appeal to their lack of a religious identity. Instead, like many other Americans whose religious beliefs and practices are increasingly shaped by partisan politics, their liberal identity — rather than a nonreligious or secular identity — may be the main characteristic many religiously unaffiliated people have in common.

But it’s possible that Sanders also may be drawing on a little-tapped well of secular enthusiasm on the left, particularly among atheists and agnostics. “Their nonreligion may not be the driving factor for their support for Sanders, but it’s very likely a contributing factor,” said David Campbell, a political science professor at Notre Dame University who studies religion and civic life. His research has found that a significant share of Democratic Party activists don’t just lack a religious affiliation, they have actively embraced a secular worldview — and political arguments rooted in socioeconomic justice might resonate especially with them. Nonreligious voters might, too, see their own identity reflected in Sanders’s lack of overt religiosity, which could add to his appeal. But there’s also room, Campbell said, for candidates to reach atheists and agnostics even more directly. “This is a big group of voters who usually get ignored,” he said. “One way to stand out from the pack is to try to explicitly address them in secular terms.”

Marshaling Democrats by invoking their secular identity, though, would be risky for Democratic politicians in a number of ways. Atheism, in particular, remains a fairly politically toxic brand. As FiveThirtyEight’s Perry Bacon Jr. noted recently, big segments of the electorate, including 28 percent of Democrats, say they wouldn’t vote for a well-qualified presidential candidate who was an atheist. So messages directed specifically at nonreligious voters could alienate the religious Democrats who make up the bulk of the party — not to mention voters in the general election. “The Democratic Party is very far from a universally secular party,” Campbell said. “It would take a very artful politician to be able to mobilize secular voters while also holding onto religious voters.”

So for the time being, the religiously unaffiliated may remain a largely invisible group within the Democratic Party. But it’s worth watching how this group’s allegiances shift over the course of the primary — because as the number of nonreligious Democrats continues to grow, candidates may need to start figuring out strategies for how to appeal to them more directly.

Laura Bronner contributed research.