Salena Zito

Among Catholics, Santorum's message could be put to the test again in Illinois on March 20 and later contests in Wisconsin and Pennsylvania, said Burt Rockman, a political science professor at Purdue University in Indiana. In earlier primaries, CNN exit polling showed Romney won the Catholic vote in New Hampshire, Nevada, Arizona, Michigan, and Florida.

“It really is stunning, and kind of interesting, that Santorum is losing Republican Catholics,” Rockman said. “Catholic-rich upcoming primary states ... are going to be a problem for his viability.”

America's Catholics largely are mainstream voters, said Catherine Wilson, a Villanova University expert on religions' impact on politics.

"As a whole, they tend to view their faith as private and are unaccustomed to discussing the content of their faith in public,” she said.

Given the progressive, moderate, and traditional strains within the Catholic vote, it is difficult to predict the outcome of November's election. Catholics constitute 25 percent of eligible U.S. voters and their vote is not a slam-dunk for either party, Wilson said.

Research shows Catholics can become the quintessential swing-voters among people of faith, Wilson said, because they hold positions on social issues that largely mirror the general population. A 2009 Gallup poll, for example, found 40 percent of Catholics and 41 percent of non-Catholics consider abortion to be morally acceptable; 67 percent of Catholics and 57 percent of non-Catholics said the same about premarital sex, she said.

Across the board, churchgoing Catholics hold more progressive views than non-Catholic churchgoers, Wilson said.

Santorum became Romney's closest competitor in the race in early February, after surprise wins in three caucus states, although political strategists largely considered those results "beauty contests" because they netted him no delegates to the Republican National Convention.

Once in the spotlight, Santorum began muddling his message with religion. He talked about contraception, adopting church doctrine against it. Then he said he wanted to “throw up” the first time he read President John F. Kennedy's speech on separation of church and state. Kennedy, the nation's only Catholic president, needed to convince voters his faith wouldn't interfere with governing.

Many Catholics tend to agree with JFK that one's faith should not be a central influence in presidential decision-making, said Wilson.

"They are interested in having a candidate run for office who is less a preacher than a real presidential contender," she said.

Santorum turned off other Catholics when he ridiculed President Barack Obama as a “snob” for saying that everyone should have a shot at a college education, said O'Rourke in Texas, which votes May 29.

"College is the American dream for ethnic Catholic parents," O'Rourke said.

Salena Zito

Salena Zito is a political analyst, reporter and columnist.