Both Tim Kaine and Mike Pence brought their faith into Tuesday night's debate, emphasizing their religious beliefs and even quoting the Bible. Both vice presidential nominees are devout Catholics, and together they are proof that Christian faith can drive the beliefs of voters across the political spectrum. So one of our listeners asked recently why the Christians of one party get so much more attention than the others:

"I'm a white 33-year-old voter and I vote based, in large part, on my Christian faith, which is precisely why I am a Democrat and a big-time Hillary-supporter. Many of my former seminary classmates and other religious folks I know also vote based on their faith and are therefore Democrats. Are there any good numbers on the Christian left vote? We hear so much about the vote of the Christian right, but we rarely hear about the Christian left, with the exception of occasionally hearing about the religious dimensions of the black vote. How do religious voters of all traditions tend to factor on the left?" — Shea, from Virginia

SeparatorShea is right that the Christian right gets a lot of attention — though this year, it may not be attention that conservative Christians are excited about. Plenty of people are wondering about the future of the Christian right, given both the shrinking population of white Christians and a Republican candidate whose words, policies, and life choices leave some Christians uneasy.

So where is the Christian left in this discussion?

The easiest question to answer here is Shea's numbers question: "How do religious voters of all traditions tend to factor on the left?" The Pew Research Center has numbers on this — not exactly about left versus right, but about the religious affiliations of Democrats and Democratic-leaners versus Republicans and Republican-leaners.

On the right, white Christians dominate, with whites from "evangelical" denominations making up the plurality of Republicans and Republican leaners. (As for what exactly "evangelical" means, that's another complicated question.) Though the numbers have shifted somewhat over the years, the Republican party has remained dominated by white Christians for decades now.

Among Democrats and Democratic-leaners, religiously unaffiliated people (including atheists and agnostics, for example) make up the largest share, at nearly one-third of those people. That's a profound shift from 20 years ago; those "religious nones" are the fastest-growing religious group in America, and they are concentrated far more among Democrats than Republicans.

Long story short: the right is very, very Christian, while the left is much less so. That Republican group is 83 percent Christian. For Democrats, it's 59 percent.

So it makes sense that the Christian right is also better-known than the Christian left. Fully 58 percent of people said they had heard "a lot" or "a little" about the Christian right, according to Pew, compared to 41 percent who said the same of the Christian left.

Furthermore, 4 percent of Americans say they agree with the Christian left, compared to 14 percent who say so of the Christian right.

To be clear, there are lots of devout Christians on the left, many of whom take their faith into account when casting their votes. But these data show that there's a difference between having lots of people and being a cohesive political force.

"None of this is to suggest that there aren't individual voters on the political left whose politics are motivated by their religious beliefs — there assuredly are," said Greg Smith, associate director of research at the Pew Research Center. "But it's tough to see much in these data to suggest that the religious left is gaining in size or power as a movement, at least among whites."

Democrats are way more religiously diverse

One big reason the Christian left (or even a broader "religious left") is hard to assemble into one movement: a broad range of beliefs.

"What's always been a challenge for the progressive side of the religious world is they're so diverse," says Robert Jones, CEO of the Public Religion Research Institute and author of The End of White Christian America. "It's a much different world. It's sort of by definition less likely that there's going to be a single leader or two that's going to speak for that entire diverse movement."

It's true that there are some prominent leaders on the left: William Barber, who spoke at the Democratic National Convention, is from the black evangelical church. Sister Simone Campbell and the Nuns on the Bus are some prominent voices among progressive Catholics. And many other progressive Christians have associated their beliefs with theologian Jim Wallis. (And then there are plenty of adherents of other religions on the left, meaning a broader "religious left" would face a similar problem in cohering together as the "Christian left" would.)

But none of the various voices on the left have quite had the clout or the unifying power that people like Jerry Falwell or James Dobson once amassed. Many of the famous leaders on the right have been similar in that they have been white and "evangelical."

All of which is to say that expecting a Christian left to resemble a Christian right is probably wrongheaded.

"I think that the Christian right was such a phenomenon in the '80s and '90s, that I think people tend to take that as a template and try to apply it to the left," Jones said. "And they say, 'Where is that thing on the left that looks like that thing on the right?'"

"That kind of thing is just not going to exist," he said.

Religion is more of a top priority for many conservatives

One more thing preventing one unitary Christian left movement from forming is that religion functions differently on different sides of the aisle:

"What we find is that among conservatives, many more of them tend to say it is the most important thing," Jones said. "Among progressives, we tend to find — even those who attend religious services fairly often — they tend to say it's one among many important things."

So while religion does motivate some liberals' votes, one might imagine that it's easier to unite conservative voters, for whom religion tends to be a higher priority.

However, Jones said, neither side exactly has a massive amount of clout.

"I don't think right now I would say there's a strong Christian right movement, nor is there a strong Christian left movement in the country today," Jones said.

Indeed, the number of Christians in the U.S. is declining quickly, having fallen from 78.4 percent of the population in 2007 to 70.6 in 2014. And white Christians (who make up the overwhelming majority of Republican Christian voters in particular) have slipped below the majority — in 2008 they were 54 percent of the population. Today, they're 45 percent.

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