Romney released an ad Thursday repeating the war on religion charge. Next week, Sandra Fluke --- a Georgetown University law student who was thrust into the national spotlight after radio show host Rush Limbaugh called her a "slut" for her role in supporting Obama's contraception rule --- will introduce the president at a stop in Denver.

Ryan's own Catholicism became a major issue this year, with the United States Conference of Catholic Bishops criticizing his proposed federal budget for what the bishops said would be its adverse impact on the poor.

The bishops cautioned against overreaching budget cuts that endanger "poor and vulnerable people." The bishops' message called on "Congress and the administration to protect essential help for poor families and vulnerable children and to put the poor first in budget priorities."

This split between politically conservative and liberal Catholics has existed for decades in the Catholic Church. But with Ryan running for vice president, some experts expect this divide to be sharpened.

"What Ryan will highlight is a division within the Catholic community," Green said. "More politically liberal Catholics are very critical of the Republican approach and the Ryan budget, but Ryan has taken them head on."

In an April speech at Georgetown, a Catholic school, Ryan defended his budget in religious terms.

"The work I do as a Catholic holding office conforms to the social doctrine as best I can make of it," Ryan said. "What I have to say about the social doctrine of the Church is from the viewpoint of a Catholic in politics applying my understanding of the problems of the day."

Ryan's $3.53 trillion budget doubles down on past proposals to overhaul Medicare and other government programs that are seen as politically sensitive. While the budget has little chance to become law, it draws a distinct contrast with Democratic views on spending.

That speech, along with other statements that put his budget into religious terms, led liberal Catholic groups to openly protest Ryan's budget.

In particular, NETWORK, a group founded by 47 Catholic nuns that speaks out on social justice issues, went on a bus tour around the country to protest the Ryan budget.