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.

"I believe in an America that is officially neither Catholic, Protestant nor Jewish," Kennedy said.

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 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 Protestants than Catholics, who have long had a network of parochial schools.

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 shortlist 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 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.

Santorum's comments were a new twist on a steady theme of his candidacy: that Obama and other Democrats have a secular worldview not based on the Bible, one they are intent on imposing on believers.

Campaigning in Iowa in December, Santorum said Obama and his allies have "secular values that are antithetical to the basic principles of our country." In Des Moines a few days later, he said the same people adhere to a "religion of self" rather than one based on the Bible.

Speaking to a group of ministers in Plano, Texas, earlier this month, Santorum argued that the left is "taking faith and crushing it."

Santorum has regularly argued on the campaign trail that Obama and his allies' views on abortion, same-sex marriage and the proper role of government prove they have distinctly secular values — and that the election offers a key and perhaps final chance for religious people to fend off their intrusions.

The relationship between religion and government has emerged as a flash point in the presidential campaign in recent days after an effort by the Obama administration to require religious institutions to include contraception in health-insurance plans for employees. All of the Republican candidates objected to the effort, which the administration tweaked after a massive outcry, especially from Catholics.

But even in a nominating process heavy on Christian themes, Santorum stands out for his comfort in embracing religion. His contention that government is intruding into religious liberty predates the Obama decision.

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.

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.

Santorum does not limit his emphasis on religion to religious settings. Last week, in a speech ostensibly on the economy, Santorum said Obama's proposal to limit deductions for upper-income taxpayers, including for charitable donations, was a directed attempt to shrink the role of churches and other civic organizations in people's lives.

"We need to create a rich society with lots of places for you to go before you go to the government for help and assistance in the problems that you're dealing with. Charities, churches. It's no wonder that the president, one of his tax proposals, sought to limit charitable contributions. They get in the way of government, you know, in providing for you," Santorum told the Detroit Economic Club.

On Sunday Santorum said on ABC's "This Week" program he doesn't believe in the separation of church and state, noting that Kennedy's speech makes him want to "throw up."

"I don't believe that the separation of church and state is absolute," Santorum said. "The First Amendment means the free exercise of religion and that means bringing people and their faith into the public square."

Santorum said Kennedy's 1960 speech in Houston about the separation of church and state, was an "absolutist doctrine" that he disagrees with.

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

"I think historically religion has been divisive when it's gotten connected with politics," said John Danforth, who served 20 years as a Republican senator from Missouri. "I think Republicans are better if they stick with the big issues and the economic issues and the power of government and don't frame it in religious terms."