Rachel Zoll, AP Religion Writer

Rick Santorum’s political good fortune in the Republican presidential primaries has come about in large part because of his appeal to evangelicals. A Roman Catholic, he is a beneficiary of more than two decades of cooperation between conservative Protestants and Catholics who set aside theological differences for the common cause of the culture war.

Doctrine — and anti-Catholic bias — once split Protestants and Catholics so bitterly that many evangelical leaders worked to defeat John F. Kennedy because of his religion. When Kennedy sought to confront suspicion about his Catholicism, he made his now-famous faith speech to the Greater Houston Ministerial Association, a group of evangelical Protestants in Texas. Five decades later, when some prominent evangelical leaders gathered at a Texas ranch to discuss backing a 2012 GOP candidate, Santorum was their choice.

Now running about even with Mitt Romney, Santorum has nearly doubled his support from white evangelical Republicans, from 22 percent last month to 41 percent two weeks ago, according to surveys by the Pew Research Center’s Forum on Religion and Public Life. An Associated Press-GfK survey conducted more recently, Feb.16-20, found Santorum leading Romney among white evangelicals, 44 percent to 21 percent. White Catholics also preferred Santorum, 38 percent to 29 percent, in the AP-GfK poll.

The high regard extends to Santorum’s personal life. His seven children have been home-schooled, a practice much more common among conservative American Protestants than Catholics, who have a network of parochial schools built over centuries. His concerns — opposing gay marriage and abortion, promoting traditional roles for women — contribute to that appeal. The Christian Post, an evangelical media outlet, published an article this week called “Catholic Politicians You Thought Were Evangelical,” with a short list of the most-often misidentified, led by Santorum.

The former Pennsylvania senator’s pointed rhetoric questioning the authenticity of other Christians can make him sound more like a preacher than a politician, but it draws support among many conservative Christians. He said recently that President Barack Obama, also a Christian, holds a “phony theology,” then insisted he wasn’t attacking the president’s faith but his environmental views. The Obama campaign condemned his remark.

Also drawing attention is a 2008 speech to Ave Maria University in Florida, a private Catholic school established by the Domino’s Pizza founder. In it, Santorum warned that Satan has been waging a spiritual war against the United States and has infiltrated academia, liberal Protestant churches and politics.

“Satan has done so by attacking the great institutions of America, using those great vices of pride, vanity and sensuality as the root to attack all of these strong plants that have so deeply rooted in the American tradition,” Santorum said, in a video posted by Right Wing Watch, a project of the liberal advocacy group People for the American Way. “We look at the shape of mainline Protestantism in this country and it is in shambles. It is gone from the world of Christianity as I see it.”

Romney, Santorum’s main rival for the nomination, struggles with conservatives not only because he once supported legalized abortion, which he now condemns, but also from distrust of Mormon teaching among some Christians. He rarely speaks directly about his faith or any other.

Bill Portier, a Catholic theologian and historian at the University of Dayton in Ohio, said many in the United States have come to identify conservative religion only with evangelicalism. A growing number are describing themselves as “spiritual, not religious” and aren’t affiliating as closely with a particular denomination.

Portier said his students at the Catholic university are often shocked to learn about a Catholic teaching on a social or moral issue that differs from a conservative Protestant view.

“It’s their default, what evangelicals say,” he said. “It kind of comes to them from osmosis through our culture.”

One of the best-known efforts to bring the two Christian traditions together came in the 1994 statement “Evangelicals and Catholics Together.” The authors were Chuck Colson, the Watergate felon turned born-again Christian, and the Rev. Richard John Neuhaus, a Lutheran who converted to Catholicism and was also often mistaken for an evangelical. In 2009, Catholics, evangelicals and Orthodox Christians again pledged their unity on moral issues in a document called the “Manhattan Declaration,” in which they promised civil disobedience if any laws are enacted that violate their conscience.

Some political veterans warn Santorum that what fires up the base can be a losing strategy in the general election.

Peter Wehner, a Republican who served three presidential administrations, most recently under George W. Bush, said in an article about Santorum that social conservatism must be discussed in positive terms, as promoting human dignity, “rather than declaring a series of forbidden acts that are leading us to Gomorrah.” Wehner is a senior fellow at the Ethics and Public Policy Center, a conservative think tank where Santorum was a fellow after he lost his U.S. Senate seat.

“A wise observer told me years ago,” Wehner wrote on Commentary magazine’s website, “that for a politician to be seen as the aggressor in the culture wars is the quickest way to lose them.”