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Rick Santorum attending a prayer service this month at an evangelical church in Puerto Rico. | Christopher Gregory/Getty Images

Cathy Willauer, who is Roman Catholic and a mother of four, says that her religion is important to her and that she shares the same values as Rick Santorum.

But Mrs. Willauer, 50, who lives in Annapolis, Md., has decided to support Mitt Romney in Maryland’s Republican presidential primary on April 3. She said she had more confidence that Mr. Romney, a former governor of Massachusetts, could better manage the economy.

Besides, she said, Mr. Romney, who is Mormon, appears more tolerant of people of other faiths.

"While my personal values may align more closely with Senator Santorum’s," she said, "I feel Governor Romney is more willing to tolerate different views and values, and the president of the United States has to accept and respect the right of every American to believe as they will."

Mrs. Willauer, who attended a Romney event in Arbutus, Md., last week, is part of a striking pattern that has emerged during the Republican primary season: more Catholic Republicans are favoring Mr. Romney even though Mr. Santorum is Catholic.

Mr. Santorum, a former senator from Pennsylvania, has trailed Mr. Romney among Catholics in 10 of the 12 states in which Edison Research conducted exit polls that asked about religion.

With two exceptions, he has lost the Catholic vote by a minimum of 7 percentage points (in Michigan, where Mr. Romney grew up) and by as much as 53 percentage points in Massachusetts, where Mr. Romney was governor. He has even lost among Catholics in the South, although he was nearly tied with Mr. Romney among Catholics in Tennessee and won decisively among Catholics in Louisiana.

In most of the primary contests, whether he has won or lost, Mr. Santorum has been buoyed by the support of evangelical Protestants. He has done best in states with substantial evangelical populations and they have become his most reliable base, along with some Tea Party supporters and those who call themselves very conservative.

In fact, many voters are unaware of his religion. A Pew survey this month found that only 42 percent of Catholic Republicans knew that Mr. Santorum was Catholic. At the same time, 11 percent of Catholic Republicans and 35 percent of white evangelical Republicans said they thought he was an evangelical.

"There’s an intensity to his statements, and to the subjects he discusses — the rise of secularism, the criticism of people of faith in the public square — that’s often associated with evangelicals," said John C. Green, a political scientist at the University of Akron who studies religious voting patterns.

Analysts see many reasons for Mr. Santorum’s lagging among Catholic Republican voters, the main one being that Catholics, who make up about a quarter of the total electorate, are not monolithic and are more representative of the electorate as a whole.

"There is no Catholic vote, per se," said Catherine E. Wilson, a political scientist at Villanova University. "They mirror the general population, with progressives, moderates and conservatives. And Santorum is winning the conservatives."

In 1960, Catholics voted overwhelmingly for John F. Kennedy. But in 2004, when Senator John Kerry of Massachusetts became the first Catholic nominee since Kennedy, he lost the Catholic vote to President George W. Bush, a born-again Christian.

In 2008, President Obama won the Catholic vote over the Republican candidate, John McCain. This year, Dr. Green said, Catholics appear likely to be divided again, with conservatives voting for the Republican nominee, liberals voting for Mr. Obama and the moderates "up for grabs."

So far, Mr. Romney, who has emphasized the economy, has been more successful in winning Catholics over than Mr. Santorum, who has emphasized social, cultural and religious issues.

An important indicator of voting preference is how often someone attends church. Those who attend at least once a week tend to be more conservative than those who attend occasionally. But only about one-third of Catholics who responded to the latest New York Times/CBS News poll said they attended church weekly.

Mr. Santorum was asked last week by Sandy Rios, a Fox News contributor hosting a program on American Family Radio, to explain why he was not winning more Catholic voters.

He said he did not understand it himself — "I really wish I could tell you," he said — but he said he thought it might correlate with church attendance.

"With folks who do practice their religion more ardently," he said, "I tend to do well."

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Ashley Parker contributed reporting from Arbutus, Md.; Susan Saulny from Mandeville, La.; and Marjorie Connelly from New York.