Tuesday's public spat between an atheist advocacy group and Christian conservatives was full of bluster and drama, all over a CPAC conference booth.

This small theater in the culture wars may be of little consequence beyond Washington, but it highlights a dynamic in which non-religious voters are gravitating steadily away from Republicans, even as Democrats have made few major efforts to galvanize their support.

Take the following chart for starters:

Before the 1990s, Republicans won as much as 43 percent of non-religious voters, but suddenly began losing them by a 2 to 1 margin just as the group began to grow. By 2008, voters with no religion had grown to 12 percent of the electorate - more than Hispanics - and Barack Obama won them in 2008 and 2012 with wider margins than any previous Democrat. Mitt Romney narrowed the gap in 2012, though at 44 points he was hardly competitive. It's important to note that most of these voters are not full-on atheists espousing intense opposition to religion on principle, but agnostic or "nothing in particular."*

Why did this shift happen? In their book American Grace, sociologists David Campbell and Robert Putnam argue that the rising number of "nones" and their increasing Democratic tilt are a reaction to the Republican Party's tightening alignment with Christian conservatives since the 1980s. In one recent example, a 2012 Pew Research Center poll found two in three religiously unaffiliated Americans agreeing that religious conservatives have too much control over the Republican Party.

The strife over atheists' role at the CPAC conference is hardly representative of the ways religious and less religious Americans get along in their daily lives. Indeed, American Atheists clearly aimed to antagonize religious conservatives in attending. But the sharp reaction from Christian conservative leader Tony Perkins to the prospect of atheists taking part reinforces the message religiously unaffiliated Americans (beyond atheist activists) have become familiar with: you're not welcome.

Democrats are not all-out winners in the deepening divide over religious lines. Exit polling found voters who attend church weekly or more - a group three times the size of the "nones" - backed Mitt Romney by 20 percentage points over Obama in 2012 (59 to 39 percent), higher than than in any election since 2000.

But the general dynamic seems unlikely to change anytime soon if parties grant their constituents' wishes. Conservative Republicans broadly see religion as an important element of government. In a 2012 Post-ABC poll, over six in 10 conservative Republicans said it matters that presidential candidates share their religious beliefs and that the country has gone too far keeping religion out of government.