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As voters in 12 mostly Southern states head to the polls this Super Tuesday, the voters most political observers have their eyes on are conservative evangelicals. They play an outsize role during the Republican nominating process and are critical to the electoral prospects of GOP candidates — more than one-quarter of Mitt Romney’s supporters in 2012 were white evangelical Protestants. In the past few GOP contests, Donald Trump has benefited from unexpectedly high levels of support from the community, flummoxing his rivals and alarming national evangelical leaders.

On the Democratic side, meanwhile, religion has seemed a nonfactor. In fact, only in South Carolina’s Democratic primary on Saturday did exit pollsters even bother to ask voters about religion — and then only to inquire how often they attend church. But by ignoring religion, pollsters and others have missed the story of a key constituency that is quietly shaping the Democratic Party’s religious coalition and the pivotal role it could play in determining the outcome of the 2016 general election.

Religiously unaffiliated voters may be the most important political group that no one talks about. Few constituencies have voted more strongly Democratic in recent elections. They represent a larger potential pool of voters than white evangelical Protestants — 23 percent of Americans are religiously unaffiliated today, while 17 percent are white evangelical Protestant. And the unaffiliated are growing at an unprecedented rate, more than doubling in size over the past two decades.

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All this should be good news for Democrats. But as Hillary Clinton begins to solidify her path to the Democratic nomination, this key constituency is remarkably ambivalent about her campaign. Only 52 percent have a favorable view of her, compared to 63 percent who view Bernie Sanders positively and 69 percent for Barack Obama. Perhaps more worrying for the former secretary of state is the fact that close to half (44 percent) of unaffiliated voters express a negative view of her.

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Will this lack of enthusiasm for Clinton doom her candidacy? No, although it’s worth noting that a majority of Vermont’s Democratic voters are religiously unaffiliated, as are pluralities in Colorado and Massachusetts (all Super Tuesday states). Lukewarm support for Clinton could, however, be a factor in determining November’s outcome.

That’s because the religiously unaffiliated, despite representing nearly one in four Americans, have had only a fairly modest political impact in recent elections. In the 2012, unaffiliated Americans made up 20 percent of the adult public, but only 12 percent of all voters. Even as they have grown in size, their share of the electorate has remained essentially flat. As a group, they are hard for candidates to reach and mobilize because they are actually a loose confederation of Americans with varying religious orientations — some are atheists or agnostics, others unattached believers, and many are spiritual-but-not-religious types. As a result, they tend to lack a cohesive issue agenda and overarching political identity. They share a liberal outlook on questions of social policy, but the unaffiliated don’t always prioritize the same issues.

And yet while unaffiliated voters may be elusive, they are not — as it turns out — unpredictable in their voting patterns. Over the past three national elections, they have become among the most reliable Democratic constituencies, backing the party nominee by at least a two-to-one margin. In 2012, Obama bested Romney among unaffiliated voters by a whopping 44 points (70 to 26 percent); overall, they made up roughly one out of every five Obama supporters. The unaffiliated now vote more solidly Democratic than Jewish voters, historically a key part of the party’s coalition.

The impact of religiously unaffiliated voters is also tied to location. Unlike white evangelical Protestants, who tend to be clustered into reliably Republican Southern states, the unaffiliated are scattered across red, blue and purple states alike. In 19 different states, they are the single largest religious group, and they make up more than one-quarter of the residents in four swing states: Nevada, Colorado, New Hampshire and Michigan. Many of these crucial states have also experienced an unaffiliated explosion in recent years. Religiously unaffiliated Iowans, for example, have increased dramatically over the past eight years as a share of the state’s population, from 15 percent in 2007 to 24 percent today.

Throughout his time in office, President Obama has repeatedly sought to broaden the Democrats’ already diverse religious coalition by acknowledging nonbelievers. He made history in 2009 by specifically referencing nonbelievers in his first inaugural address: “We are a nation of Christians and Muslims, Jews and Hindus — and nonbelievers.” And every year since 2010, Obama has specifically included nonreligious Americans in his annual Religious Freedom Day Proclamation, writing that year: “It was the genius of America’s forefathers to protect our freedom of religion, including the freedom to practice none at all.”

Eric Sapp, a political consultant who has worked with Democratic candidates to reach religious voters, cautions that Democrats need to be careful not to stereotype unaffiliated voters. “The assumption is often that these are all secular atheists who aren’t value-driven,” says Sapp, who has also worked on campaigns to organize campuses around environmental issues, hunger and poverty. “In fact, they’re very moved by discussions of morality.”

With American attitudes toward nonbelievers and atheists softening and the political importance of unaffiliated voters increasing, it will only be a matter of time before Democrats welcome them with open arms. Whether the party likes it or not, religiously unaffiliated voters are now a critical part of the Democratic base. And for Clinton, this could be a crucial point. Her appeal will come into much starker relief if she is being evaluated not against the unapologetically progressive Sanders, but against the unabashedly conservative Rubio or Cruz.

In the early stages of the 2012 Republican primary, Mitt Romney was not highly regarded among white evangelicals, but once he became the party’s nominee, his stock rose sharply. Clinton may trace a similar path among the unaffiliated. There may not be a simple formula for turning them out, but that doesn’t mean that unaffiliated voters can’t fundamentally alter the outcome of the election this year — either by turning out to support the eventual Democratic nominee, or by deciding to stay home.