Marco Rubio’s plan for the final days before the Iowa caucuses, according to The Washington Post, is to talk about God … a lot. And with all the focus on the evangelical Christian vote in Iowa — they make up nearly 60 percent of Republican caucus-goers and are credited with powering Ted Cruz into the top tier of the race — Rubio’s plan isn’t surprising.

But the media’s focus on evangelical Christians misses the larger story: The best predictor of vote choice, according to work by political scientists Robert Putnam of Harvard and David Campbell of Notre Dame, is religiosity, not religious affiliation; Putnam and Campbell call it the “God gap.”

Religiosity, as the two describe it, includes the three B’s: belonging, behaving and believing. The stronger a person’s sense of belonging, the more frequent her church attendance and prayer, and the stronger her belief, the greater her religiosity is. (I’m focusing mostly on Christians in this article, as there is much less data on religious groups, such as Jews and Muslims, that make up relatively small shares of the U.S. population.)

This “God gap” is relatively new. As the scholars observe, “American history teaches us that religion is neither exclusively left nor right, progressive nor conservative.” Religion was invoked on both sides of the slavery debate in the 19th century, and it was a vital piece of both Prohibition and the progressive movement.

The height of religiosity in America was in the post-World War II era. The surge of churchgoers in the 1940s and ’50s had no “partisan political cast.” Instead, they were young veterans, many of them college-educated because of the GI Bill, raising families and looking to bring meaning and structure to their lives. But in the past few decades, as Putnam and Campbell write, “how religious a person is had become more important as a political dividing line than what denomination he or she belonged to.”

Pew Research Center has found the same thing, showing that during the 2014 midterm elections, frequency of religious service attendance was an increasingly good predictor of casting a Republican vote. (One caveat that Putnam and Campbell and Pew note: Black Protestants are an important exception. While highly devout, they are also committed Democrats. This exception also illustrates that these findings are generalizations and not iron-clad rules for individuals.)

There have been some changes in patterns of religious identification and religious service attendance over the past 30 years. In the 1970s and ’80s, fewer people identified themselves as mainline Protestant, while an increasing number identified as evangelical. However, by the 1990s, this denominational shift was over. What continued to change was the frequency of attendance at religious service. As Putnam and Campbell write, “what the evangelical churches have lost in adherents over the last two decades has mostly been made up for by the evangelicals’ zeal.”

At the same time that religious Americans became more religious, younger Americans increasingly identified as nonreligious. As Pew found, this growing group of “nones” — those who claim no religious affiliation — are largely part of the millennial generation and, most importantly in terms of the political landscape, are the single largest group in the survey to identify as Democrats:

And “evangelicals” are the largest group to identify as Republicans:

Putnam and Campbell argue that this kind of religious polarization is not the natural state of American politics. Instead, the political scientists suggest that it is the conscious crafting of religion and politics by opinion leaders that has led us to this place. By prioritizing issues such as abortion and gay marriage, Republicans have been able to appeal to religious people of many denominations. For example, as the first Catholic to successfully run for president, Kennedy won an overwhelming 78 percent of the Catholic vote. The next Catholic to run as the Democratic standard bearer, John Kerry in 2004, managed only 47 percent of the vote, as social issues took center stage. Further evidence of the phenomenon, which Putnam and Campbell call “the coalition of the religious” was evident Jan. 22 when evangelical Protestants joined Catholics in the “March for Life” in Washington. As Putnam and Campbell write, “What makes the current period unusual is the church-attending evangelicals and Catholics (and other religious groups too) have found common political cause.”

As the primary season moves on, appeals to religious voters will vary on a state-by-state basis. For example, as Gallup points out, New Hampshire is the second-least-religious state in the union after Vermont. But Republicans are likely to continue their appeal to the most religious segments of the national electorate because this is a reliable voting bloc for them. Hillary Clinton and Bernie Sanders, meanwhile, have little reason to worry about alienating religious voters compared with Democratic candidates 10 or 20 years ago.

Religion itself, and not denomination, has become one of the central dividing lines in American politics, and that’s likely to stay true throughout the 2016 election.