Divided Catholic electorate fixated on tonight's vice presidential debate between Joe Biden and Paul Ryan

The vice presidential debate has rarely moved the dial much in presidential campaigns, but tonight's showdown between Vice President Joe Biden and Congressman Paul Ryan might change that as their performance could sway the voting bloc that shares the candidates' Catholic faith.

The Catholic vote has been a predictor of presidential politics since the early 1990s. President Barack Obama captured 54 percent of it in 2008, but coming into Thursday the race remains tight among Catholics.

"The 'Catholic vote' is back to 'too close to call' with President Obama leading among all Catholic registered voters (49 percent Obama to 45 percent (Mitt) Romney) and Gov. Romney leading among all likely Catholic voters (50 percent Romney to 44 percent Obama)," wrote Mark Gray, who crunched the poll numbers at 1964, a research blog for the Center for Applied Research in the Apostolate.

"It's unprecedented for two Catholic candidates to face off in a national forum that is likely to be so heavily watched," Gray wrote. "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."

A Reuters/Ipsos Poll conducted last weekend shows that Catholics are divided equally between Obama and Republican challenger Romney, with each winning just under 40 percent of voters.

On Thursday, the Pew Forum on Religion & Public Life released a new analysis of the Catholic vote that found most subgroups of Catholics reliably vote either Republican or Democrat. It's only white Catholics who identify as moderates that have been divided in recent elections.

Both campaigns are mobilizing their Catholic voters to attend debate watch parties, where obersers say attendees will draw distinctions on how each candidate remains true to their faith.

"Ryan and Biden speak proudly and often about their Catholic faith, and are quick to explain how it informs their policy positions," wrote Daniel Burke and David Gibson for Religion News Service. "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 Biden 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."

On Tuesday, more than 100 Catholic theologians and scholars sent a letter to both candidates, but particularly targeted Ryan, urging them to heed the church's social teachings.

"The organizers of the statement, part of a group called On All of Our Shoulders, said that while Catholic politicians who support abortion rights are often called out for their differences with their church, 'Catholic Republicans who dissent from church teaching on issues such as torture, war, the environment and the economy receive far less scrutiny,'” reported CNN's Belief Blog.

Meanwhile, U.S. Catholic Bishops have campaigned heavily against the Obamacare mandate, which requires Catholic schools and hospitals to carry insurance that provides birth control, forbidden by church doctrine, for free.

Joseph Cella, who leads Catholic outreach for the Romney-Ryan campaign in Michigan, told RNS that Biden and Ryan can't avoid discussing issues of importance to Catholics tonight.

“I don't 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.”