By Josh Levs, CNN

(CNN) - The race for the Republican presidential nomination is on track to break new ground: For the first time in modern political history - some say ever - the GOP nominee could be someone who is not a Protestant Christian.

Front-runner Mitt Romney is Mormon, as is Jon Huntsman. Rick Santorum and Newt Gingrich are Catholics.

The only two Protestants in the race are Rick Perry and Ron Paul. Paul had strong finishes in the nominating contests so far but most political experts and Republican establishment figures say he is not favored to win the nomination ultimately. Perry has finished near the end of the pack so far but is hoping for a strong finish in the next-in-line South Carolina primary.

Neither major party has ever had a Mormon nominee. John F. Kennedy, a Democrat, was the only Catholic president.

Democrats have also nominated John Kerry, a Catholic, and Michael Dukakis, who is Greek Orthodox, but the overwhelming majority of Democratic presidential nominees have been Protestant.

Experts who follow the intersection of religion and politics say this year’s crop of Republican candidates reflects the changing electorate, the lasting significance of a Supreme Court decision, and shifting forces within American Christianity.

“Catholicism has been almost fully absorbed into the American mainstream,” says William Galston, senior fellow with the Brookings Institution.

While Kennedy faced questions from some voters over whether he would take orders from the pope, that kind of skepticism is virtually unheard of today, Galston says.

“The more interesting question is Mormonism. Because in many Protestants’ eyes, Mormons today stand roughly where Catholics did 60 years ago. They are suspect.”

But Romney, with his “unblemished personal life,” is in a unique position to help guide Mormonism into the mainstream of American politics, Galston says.

Presidential historian Douglas Brinkley says Americans have achieved enough comfort with Mormonism to make room for a possible Romney presidency.

“Are we ready for a Mormon president? I think the answer is yes,” Brinkley says.

The Mormon population is growing quickly, and more and more people have Mormon friends, he says. “It’s no longer a fringe group growing up. It’s a powerful and important religion.”

Mormons have been recruiting Southern Baptists and Methodists to join their fold, making inroads in communities across the country and raising money, Brinkley says. “The Mormon Church is booming when some of the other denominations are struggling for cash and converts.”

Mark Silk, professor of religion in public life at Trinity College, says most American voters are “prepared to think about people who are not Protestant to be president.”

The GOP field of candidates this year is “mostly happenstance” – the contenders did not rise to the front of the pack because of their religions, Silk says. But the fact that their faiths don’t seem to be hampering their chances shows “real growth in the acceptance of religious pluralism since World War II.”

There’s also a broad political force helping bond voters across different denominations.

“In the past generation, denominational differences or religious differences have become less important than the split between modernism and traditionalism within each religion,” says Galston.

“So at this point, traditional Mormons, evangelical Protestants and conservative Catholics have more in common with one another politically than they do with the more liberal elements within their respective churches.”

That break has been furthered as the issues that guide many voters’ decisions have changed over the past few decades.

“One of the big things that’s happened since the 1970s is that a lot of cultural issues have moved from the private realm to the public stage,” Galston says. “That’s happened whether it’s been abortion or gay marriage or the treatment of private schools by the IRS.”

It’s happened “much more explicitly on the conservative side than it has on the more liberal side,” Galston says.

The Supreme Court’s 1973 decision in Roe v. Wade, which said women have a constitutional right to an abortion, was a turning point.

Before that ruling, Catholics were a solid, reliable Democratic voting block, “one of the most powerful constituencies in the Democratic party,” says Brinkley.

The Vatican opposes abortion rights. And as the Democratic Party became largely supportive of the Supreme Court’s decision, the Republican Party won over Catholics who disagreed with it.

“It turned a lot of Catholic groups from Democratic to Republican,” Brinkley says. “It flipped them.”

People within each denomination who support abortion rights and take liberal stances on numerous issues, meanwhile, have formed similar bonds on the Democratic side, with religious denominations themselves playing little role, the analysts said.

About half the U.S. population is Protestant. The American Religious Identification Survey from Trinity College, published in 2009, found Protestants are 51% of the U.S. population, while Catholics are 25%. Mormons are at 1.4%, just behind Jews at 1.8%. Muslims comprise 0.3% of the population.

While a Mormon or Catholic nominee would be a first for the GOP, there’s some disagreement over whether he would be the first “non-Protestant” ever, or just the first in generations.

A December article for rollcall.com said “Gingrich’s nomination would make him the first non-Protestant to be nominated for president by the GOP.” A 2000 Slate article headlined “The Protestant Presidency” said Kennedy was the only non-Protestant “ever elected president.”

But Silk noted that it isn’t clear exactly how to characterize Abraham Lincoln’s religious affiliation.

The first Republican president “didn’t belong to any church, wouldn’t have described himself as a Protestant,” Silk said. At the same time, Lincoln expressed a deep belief in a God who is active in history.

Adherents.com keeps a list of the presidents’ religions. Four presidents were Unitarians, a movement that grew our of Protestant Christianity. Two presidents were Quakers, a group that is connected to Protestantism.

While the analysts CNN spoke to agree that the GOP field this year reflects the country’s religious pluralism, it remains centered only on Christian denominations, setting aside the question of whether Mormonism fits a traditional definition of Christian.

Just how much of a chance a candidate of another religion would have at the presidency is another question.

Some believe that Joe Lieberman, the Democratic vice presidential nominee in 2000 who ran for the party’s nomination in 2004, was not hampered by being Jewish. “I don’t think that the classic triad Catholic-Protestant-Jew makes a difference at all,” said Galston. “Joe Lieberman’s candidacy foundered, but not because he was Jewish.”

But there has never been a Jewish presidential nominee. And just how a Muslim, Hindu, Buddhist, or member of any other religion would fare is another question.

For some voters, the denominations of the candidates continue to be a relevant factor, the analysts said. Last May, a survey by the Pew Forum on Religion and Public Life found that about one-third of white evangelical Protestants would be less likely to vote for a Mormon.

In Iowa, CNN entrance polls show that born-again or evangelical Christians supported Santorum, a Catholic, well over Romney.

In New Hampshire, CNN exit polls from the Republican primary show that Catholics and Protestants both chose Romney over the competition. More Catholics – like voters in general - supported the two Mormon candidates, Romney and Huntsman, than the two Catholic candidates, Gingrich and Santorum.

Paul, for his part, came in second in New Hampshire, and placed second among Protestants and tied with Huntsman for second among Catholics.

Analysts agree that a candidate who does not believe in God would be quickly rejected by voters nationwide – even if he or she was raised Christian.

“Whether anyone would accept a professed out of the closet atheist, no,” said Galston. “You’d probably have a better chance as a former member of the Taliban.”

Weigh in on this story at Facebook or Twitter.