President Barack Obama's multiple references to Planned Parenthood in the second presidential debate showed how keen he is to sway female voters in swing states.

He may have emboldened another demographic in the process, without making an overt appeal. I'm talking about secular Americans. Compared with their religious peers, this bloc of Americans is much more inclined to support women's rights and gender equality. And the number of nonbelieving Americans is rapidly increasing.

A study released last week by the Pew Research Center provides evidence of what many sociologists have observed: The proportion of Americans who say "none" when asked their religion is the highest recorded in such surveys.

The numbers are striking. In 1990, only 8 percent of Americans claimed to have no religion. Today, about 20 percent claim as much. More than one-third of American adults younger than 30 are now religiously unaffiliated, which means that among 20-somethings, secular Americans far outnumber evangelical Christians — a big shift from 25 years ago.

The overwhelming majority of "nones" are content with their lack of religious involvement; 88 percent say they aren't interested in or looking for a religion that might be right for them.

Most religiously unaffiliated Americans aren't atheists or agnostics in orientation, yet a sizable proportion are — somewhere from one-third to one-half. Thus, the rise of the "nones" simultaneously indicates an increase in atheism and agnosticism in America.

There are several noteworthy demographic patterns. Men are more likely to be secular than women, on average. Asian- Americans, Jews and non-Hispanic whites exhibit higher rates of nonbelief than Hispanic and black Americans. The nonreligious are most highly concentrated in the Northeast and Pacific Northwest — Portland, Ore., was recently designated the "least Christian" city in the United States by the Association of Statisticians of American Religious Bodies.

What is perhaps most politically significant when it comes to the rise of irreligiosity is the strong correlation in the U.S. between being nonreligious and being left-leaning politically. Of course, as Ayn Rand fans know, many secular Americans are conservative or libertarian. But most are not.

According to the Pew study, nonreligious Americans are about twice as likely as religious Americans to describe themselves as politically liberal, rather than conservative. And they are much more likely to vote Democratic; 63 percent of nonreligious Americans support the Democratic Party, with only 26 percent supporting Republicans.

In the 2008 presidential election, three-fourths of the nonreligious supported Obama, with only 23 percent supporting Republican Sen. John McCain. A related study recently conducted by the Council for Secular Humanism found that rates of left-leaning political orientation are even higher among those Americans who affiliate themselves with secular groups. For example, among subscribers to the secular humanist magazine Free Inquiry, 75 percent label themselves as liberal, progressive or socialist, with only 7 percent self-labeling as moderate, and 3 percent as conservative.

Numerous additional studies reveal the contours of the secular-liberal connection: Nonreligious Americans, when compared with their religious peers, are more supportive of same-sex marriage and abortion rights, more interested in protecting the environment, and more supportive of creating paths to citizenship for illegal immigrants. They are less likely to support the death penalty.

Yes, you could have guessed this already. And why should political operatives care, since these voters are concentrated in uncontested blue states? As the microtargeting boom has demonstrated, small subsets of voters in swing districts and states matter, and if secularism is on the upswing, then those subsets might be growing too. It is unclear how strong a wind this is for the political left.

The rise of the "nonreligious" is partly a result of the decline of liberal Christianity. People who might have considered themselves mainline believers a generation or so ago don't want to be associated with a belief system that they think has been hijacked by the religious right. The religious liberals have become nonreligious liberals.

Secular Americans aren't an organized lot with a clear political agenda. Unlike religious voters whose convictions often determine their political choices, people are identified by the lack of something — belief in God, faith in Jesus, interest in church. That doesn't always translate into a predictable voting strategy.

In the swing state of Colorado, for example, secular women who favor abortion rights might not vote for Obama because other issues could be of greater concern to them, such as the economy.

While a few organizations, for example the Freedom From Religion Foundation and the American Humanist Association, provide support to the nonreligious, there is nothing on the secular left akin to the hundred-million-dollar media empires and multimillion-dollar PACs of the religious right. Thousands of state and national legislators don't owe their jobs to secularists.

But if the rise of the "nones" keeps up the pace it is making now, who knows what will happen.

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Phil Zuckerman is a professor of sociology and secular studies at Pitzer College in Claremont, Calif. He is the author of "Faith No More" and "Society Without God."