(RNS) When Joe Biden and Paul Ryan face off in the vice-presidential debate on Thursday night, it will mark the first showdown of its kind. For the first time in U.S. history, two Catholics appear on opposite sides of major party tickets.

A “Catholic Thrilla in Manila” as a Washington Post headline put it, recalling the famous 1975 Ali-Frazier heavyweight bout in the Philippines. Store window signs in the host city of Danville, Ky., prefer the “Thrill in the Ville.”

Whatever it is called, expectations among Catholics run as high as the stakes for both campaigns.

Joseph Cella, who leads Catholic outreach for the Romney-Ryan campaign in Michigan, where the GOP ticket has nearly closed a 10-point gap, said the campaign is organizing debate-watching parties nationwide.

“I don¹t see how Vice President Biden and Congressman Ryan could avoid discussing principles of importance to Catholics,” said Cella, a veteran conservative activist. “I don¹t think their faith will dominate the debate, but it will be an important component, and will offer sharp illustrations of the differences between the Obama administration and Gov. Romney.”

Ryan and Biden speak proudly and often about their Catholic faith, and are quick to explain how it informs their policy positions. But it’s not clear that either man will try to use those religious bona fides to gain an advantage in tonight's debate.

The problem is that while Ryan and Bien want to try to win over the critical Catholic vote, both candidates have contradicted the Catholic hierarchy at times – Biden over abortion rights and gay rights, Ryan over budget plans that critics say contravene Catholic social justice principles on the common good and caring for the poor.

“They are both vulnerable,” on that score, said Maria Mazzenga, a scholar at Catholic University in Washington, because neither man follows the full slate of Catholic teachings. “In that sense, they are much like their Catholic viewers."

At the same time, as vice-presidential nominees, Biden and Ryan are far from average Catholics. They are candidates nearing the tail end of a very tight race, where small mistakes can be magnified and can lead to sudden shifts in the polls. “They don't want to appear to be contradicting Catholic teachings” on such a prominent stage during such a close election, Mazzenga said.

That hasn’t stopped other Catholics from engaging in some pre-debate wishing, and spinning.

On Tuesday, more than 100 moral theologians, Catholic economists and scholars released an open letter critiquing Ryan’s libertarian leanings as contradictory to Catholic teaching – an effort to balance the view that a Catholic candidate's positions on abortion and gay marriage should trump all other considerations.

“If the Catholic question is asked, I think it will not be about social issues or contraception, but about citizenship,” said Stephen Schneck of Catholic University and a co-chair of Catholics for Obama. “When Biden talks you can almost hear that old Catholic way of thinking when he speaks words like duty and service, and when he talks about what we owe one another as Americans.”

Schneck said Biden, who will surely note that he was born in a blue-collar family in Scranton, Pa., is more of an old-school Catholic who has “a way of framing things that grew out of our old Catholic neighborhoods when our role models were nuns, cops, and firefighters.”

Ryan's style of Catholicism, he said, is less ethnic and more suburban, and that is reflected in their respective economic policies. In that sense, the two men perfectly reflect the demographic and generational divide among Catholics, who have gone from a reliably Democratic, working-class bloc to a key swing vote with constituencies in white, upper middle class suburbs and in Latino immigrant communities in the cities – and most everything in between.

What both sides agree on is that much is at stake. Catholics comprise nearly one-quarter of the electorate, and Obama’s once-growing lead among Catholics has shrunk dramatically, as has his edge in many categories since his dismal performance in the first presidential debate.

At the Center for Applied Research in the Apostolate at Georgetown University, researcher Mark Gray crunched the numbers this week and found Obama barely leading among all Catholic registered voters, 49-45 percent over Romney, but Romney leading 50-44 percent among all likely Catholic voters.

“It's unprecedented for two Catholic candidates to face off in a national forum that is likely to be so heavily watched,” Gray wrote on his blog, Nineteen Sixty-Four. “It will be interesting to see if faith becomes part of the discussion and if the debate moves the Catholic vote one way or the other.”