Americans are losing their religion at an unprecedented rate, with fully one-quarter of adults saying they belong to no church or religious institution, according to a new survey from the Public Religion Research Institute. These religiously unaffiliated adults are more numerous than members of any single church or religious group.

The findings dovetail with a Pew Research Center survey published earlier this year that found that the non-religious are now the largest religious voting bloc, outnumbering political heavyweights such as Catholics and white evangelical Protestants when it comes to registered-voter numbers.

But the PRRI survey explains why it seems highly unlikely that the unaffiliated will be a potent force at the ballot box this fall: Many of the religiously unaffiliated don't bother to vote.

This chart sums it up pretty well. In 2008, the unaffiliated made up 17 percent of the total population but only 12 percent of the electorate. A little apathetic, perhaps, but not too shabby overall.

Since then, the unaffiliated's share of the population has risen steadily. They made up 18 percent of the population in 2010, 20 percent in 2012 and 22 percent in 2014. But as a share of the electorate, the unaffiliated didn't increase at all over the same period.

In other words, more people than ever are saying they are unattached to any particular church or religion. But percentage-wise, fewer of them are actually going out to vote.

It's important to note that there is a lot of diversity within the ranks of the unaffiliated. Some are atheists, some are agnostics, and some just don't care enough to go to church.

It's instructive to compare the unaffiliated with white evangelicals, the highly conservative religious group that has long been an influential force in Republican politics.

In 2012, there were just as many white evangelicals as unaffiliated people among the general population. But evangelicals "accounted for more than one in four (26%) voters because of higher voter registration and turnout rates," according to the PRRI report. Because of that, they wielded more than twice as much clout in that election as the unaffiliated.

Why don't the unaffiliated vote? As Pew Research Center's Greg Smith told me earlier this year, "It could be the 'nones' are not connected, almost by definition, to religious institutions, which can play an important role in spurring turnout and interest in politics."

It's also the case that the unaffiliated tend to be younger than the population as a whole. And younger people in general are less likely to vote than their older peers.

It's not clear whether this year will be any different. Certainly, the earlier Pew survey indicated that the unaffiliated are at least registering to vote at a rate commensurate with their total population numbers. But the biggest hurdle for many of these voters seems to be showing up on Election Day.