For the past four decades, the notion that religious beliefs should guide voters’ decision-making has been largely monopolized by the Republican Party. But the partisan “God gap” hasn’t gone unnoticed by some religious Democrats, who have urged candidate after candidate to make appeals to religious values and beliefs in the hope of turning the “religious left” into a politically relevant force. And as the 2020 Democratic primary ramps up, there’s already speculation that the right candidate could tap a long-dormant reserve of religious energy among Democratic voters.

First Cory Booker — who was literally anointed by his pastor ahead of his presidential announcement — was touted as a possible candidate of the “religious left.” Then Pete Buttigieg stepped in to claim that mantle, telling reporters that the left “need to not be afraid to invoke arguments that are convincing on why Christian faith is going to point you in a progressive direction.” Meanwhile, several other presidential hopefuls, including Elizabeth Warren, Kamala Harris and Kirsten Gillibrand, are all talking openly about their religion on the campaign trail, even making arguments for why their policy positions — whether it’s abortion rights or income inequality — are linked to their faith.

And to some extent, forging connections between faith and politics makes sense for Democratic candidates — a majority of Democratic primary voters are religious. But there are several big hurdles facing any Democrat looking to use the language of faith to marshal voters in the primary. For one thing, the Democratic coalition isn’t dominated by a single religious group. And Democrats don’t prioritize religion the way Republicans do — in fact, the Democratic Party has been growing steadily less religious over the past 20 years. Certain groups of religious voters — in particular, black Protestants — will likely play an important role in the primary, and there may be some room for candidates to appeal to religious moderates. But in a diverse and increasingly secular party, religious rhetoric alone may not get the candidates very far.

Democrats are religious, but religiously diverse

Religious Democrats may not get as much attention as their counterparts on the right, but that doesn’t mean they don’t exist. About 65 percent of Democratic primary voters in 2016 reported having some kind of religious affiliation, compared to 84 percent of Republican primary voters. But as the chart below shows, religious voters in each party may not have much else in common. Republicans are fairly racially and religiously homogeneous: In 2016, the vast majority (70 percent) of Republican primary voters were white Christians, according to the Cooperative Congressional Election Study. Religious Democrats, by contrast, are much more diverse — 31 percent are white Christians, 22 percent are nonwhite Christians, and 12 percent belong to a non-Christian religious group (Jews, Muslims, Hindus, etc.) or say that their religious affiliation is “something else.”

The result is that Democratic candidates are trying to reach a smaller and more splintered religious audience than Republican candidates are targeting in their own primary. “Talking about religion is a much more complicated task when you’re trying to simultaneously address white Catholics and black Protestants and Muslim and Jewish Americans,” said Robert P. Jones, CEO of PRRI, a research organization that studies religion and politics. “They may not have all that much in common, other than the fact that they identify as religious, which makes them harder to appeal to and organize.”

And while talking about religion can be a good strategy for gaining media attention, there’s little evidence that it’s translating into actual gains among religious voters — at least, not yet. A Morning Consult tracking poll conducted May 20-26 among Democratic primary voters found that Joe Biden, a Catholic, has a commanding lead among all major religious groups, followed in all but one case by Bernie Sanders, who may be the only candidate in the race to say he doesn’t participate in organized religion.

“It’s hard to go up against Biden because he appeals to moderate Catholics and Protestants — he’s from their world,” said Ryan Burge, a political science professor at Eastern Illinois University who studies religion and politics. And according to the 2016 CCES survey, moderate Democratic primary voters are more likely to be religious than their liberal counterparts, so if Biden is also appealing to moderates, that could compound the challenge for his opponents. “If Biden is capturing most of the moderates, there just aren’t that many religious voters left to scoop up,” Burge said.

Democrats have gotten a lot less religious

And even though a substantial number of Democrats are religious, they have come to make up a smaller and smaller subset of the party. Over the past two decades, the share of people in the Democratic coalition who don’t identify with any religion doubled, from 14 percent in 1998 to 28 percent in 2018, according to the General Social Survey. The result is that today’s Democratic Party is increasingly secular, which complicates and limits traditional forms of faith outreach. “This emerging group of secular Democrats coexists a little uneasily with the more religious wing of the party,” said David Campbell, a political science professor at Notre Dame and the coauthor of “American Grace: How Religion Divides and Unites Us.” “It’s a sizeable portion of the electorate to ignore, but I think the party has yet to figure out how to appeal to these people.”

Now to be clear, most of the religiously unaffiliated don’t reject religion outright, so candidates who focus on faith may not run any serious risk of alienating these voters. In fact, according to the 2016 CCES data, only 9 percent of Democratic primary voters said they were atheists, while 8 percent said they were agnostics and 18 percent identified as “nothing in particular.” And notably, voters who fell into this last category were still surprisingly connected to organized religion. About half of these Democrats said they still attend church occasionally, and 37 percent said that religion is at least somewhat important in their lives.

However, the fact that Democrats are becoming less religious does mean that religiously-based appeals might not take candidates very far in the primary, or at least not as far as they once might have. Plus, like so many other aspects of our personal identities, there is evidence that Americans’ religious selves are increasingly shaped by our partisan allegiances, with Republicans becoming more religious and Democrats less so. Michele Margolis, a political science professor at the University of Pennsylvania and the author of “From Politics to Pews: How Partisanship and the Political Environment Shape Religious Identity,” found that white Democrats are drifting away from religion because of their politics, which means religion may not be as influential politically as it was in the past. “Religion hasn’t evolved to be a cue for religious voters on the left the way it has for religious voters on the right,” Margolis said. “If you live in a world where being a Democrat is equated with being less religious, and religion also isn’t central to your life, why should someone using religious rhetoric appeal to you?”

Religion may not rule Democrats’ vote choice

If there remains an obvious opportunity for some version of the religious left to emerge, it would be among black and Hispanic Democratic primary voters, who were significantly more likely than white Democrats to say that religion is somewhat or very important in their lives in the 2016 CCES survey.

And black Protestants are already quite powerful in the party. As FiveThirtyEight editor-in-chief Nate Silver wrote earlier this year, black voters (who are overwhelmingly likely to be Christian) constitute about one-fifth of the Democratic electorate and have a long and deep alliance with the Democratic establishment, making them a key constituency in the primary. According to the CCES, the vast majority of black Protestants and nearly three-quarters of Hispanic Catholics voted for Hillary Clinton in 2016.

And while it’s possible to imagine some kind of religious coalition emerging among Democrats of color, there aren’t any obvious issues that could unify black and Hispanic voters who are driven by their religious convictions, the way that abortion and same-sex marriage united white Protestants and Catholics on the right. Campbell also pointed out that many white Christian conservatives are motivated by a shared sense of religious embattlement or alienation — or the idea that their Christian values are being shoved to the margins or stamped out entirely by a rising tide of secularism. “They’re driven to get involved in politics because they see their Christian identity and Christianity’s place in American life as being under attack,” he said. “On the political left, certainly there’s a lot of talk of values being under attack, but it’s not framed in terms of an existential threat to your religious identity.”

But Democrats still ignore their party’s most religious voters at their peril, said Michael Wear, who directed faith outreach for Barack Obama’s 2012 campaign. He and other Democratic faith advisers have criticized Hillary Clinton’s campaign for failing to engage seriously with religious communities like white Midwestern Catholics or black Protestants. But he added that he’s waiting to see whether the 2020 candidates start building up an infrastructure for reaching religious leaders and groups. “Rhetoric can be powerful, but you also need relationships and outreach,” he said. “You can’t just talk about your religious identity on TV.” This outreach, Wear said, has to be careful and sincere. As even for highly religious Democrats, religion is still just one factor among many they’ll use to choose a candidate.

As the campaign continues, we’ll learn more about the candidates’ approach to faith — especially whether they prioritize outreach to religious voters in states like Iowa and South Carolina, where religion is likely to be a more important issue than in a relatively secular state like New Hampshire. But while mobilizing specific subgroups of religious Democrats will still be important, the dream of building a cohesive religious voting bloc on the left looks more distant by the year. Democrats may not have much to lose by talking about faith and values — but it may not offer them much of a reward among primary voters either.

CORRECTION: (May 29, 2019, 12:35 p.m.): An earlier version of the third chart in this story used incorrectly sized bars for the “White” and “Other” categories, though the numbers displayed were correct. The chart has been updated.