In selecting Paul Ryan for his running mate, Republican presidential hopeful Mitt Romney has made modern political history: a major party ticket with no Protestant Christian.

Some historians call it the first ever. Others say it's technically the first since Abraham Lincoln. And there is an argument to be made regarding Dwight Eisenhower.

But in any case, "this Republican ticket really symbolizes the passing of an era," said William Galston, senior fellow with the Brookings Institution.

Romney is Mormon. Ryan is Catholic.

It's a trend also reflected in the Supreme Court. Once dominated by Protestants, there are now none among the nine justices, Galston noted. "All the groups that make up the new American population, as opposed to the population of 50 years ago, are now participating on equal" terms, in politics and American society in general, he said.

"It's quietly dramatic."

For months, it appeared likely that the GOP would choose a non-Protestant for its presidential nominee for the first time in modern history. But the number two slot remained in question.

In choosing a running mate who is Catholic, Romney showed that he is not worried about damaging support among Protestants, "especially those who don't think of themselves as evangelicals," Galston said. "I'm not saying that Romney thought he could afford to take them for granted. But clearly he felt that he could reach out in another direction."

Half the U.S. population identifies as Protestant, while a quarter identifies as Catholic, according to the American Religious Identification Survey from Trinity College, published in 2009. Mormons are at 1.4%, just behind Jews at 1.8%. Muslims comprise 0.3% of the population.

The electorate has reshaped over the last several decades, including along religious lines, Galston said. Conservatives within each denomination have built alliances over core issues that outweigh denominational differences.

"The old lines between Catholics and Protestants which were dominant in our politics as recently as the 1960 election have been replaced by new lines of division, not between denominations but within them. So you now have a coalition of evangelical protestants and traditionalist Catholics, and Orthodox Jews as well. ... It is a very important fact about American politics today."

In 1960, John F. Kennedy faced questions from some voters concerned he would take orders from the pope. That kind of skepticism is virtually unheard of today, Galston said.

Abortion -- particularly the Supreme Court's 1973 decision in Roe v. Wade that women have a constitutional right to an abortion -- played a key role in building the new alliance among conservatives of different denominations, Galston said.

"From the standpoint of conservative Republicans, the fact that Paul Ryan is down-the-line pro-life is much more important than the fact that he's a Catholic rather than a Protestant," he said.

People within each denomination who support abortion rights and take liberal stances on numerous issues, meanwhile, have formed similar bonds on the Democratic side.

Romney's success in the battle for the presidential nomination also reflects how times have changed.

Neither major party has ever had a Mormon nominee.

Mormons consider themselves a Christian religion, but a restoration of the early church and therefore distinguished from Protestant tradition. Some Christians do not see Mormonism as a part of Christianity.

Protestantism began as a movement breaking off from the Catholic Church in the 16th century.

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 said in an interview with CNN in January.

The Mormon population is growing quickly, and more and more people have Mormon friends, he said. "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 said. "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, said most American voters are "prepared to think about people who are not Protestant to be president."

On the Democratic side, President Barack Obama is Protestant, while Vice President Joe Biden is Catholic. Obama's race, of course, distinguishes him in another way from the white Anglo-Saxon Protestants who have nearly always inhabited the White House.