A widening rift between House Budget Chairman Paul Ryan (R-Wis.) and Catholic bishops and activists over federal spending captures the power and unpredictability of the Catholic vote this year.

Recent polls suggest that Catholics, who make up about a quarter of the electorate, are up for grabs in the presidential race. A Gallup poll released Wednesday shows President Barack Obama leading presumed GOP nominee Mitt Romney among Catholics 51 percent to 45 percent.

But a survey published last week by the Pew Research Center for the People and the Press shows Catholics favoring Romney by virtually the same margin, 50 to 45 percent. In the 2008 presidential election, Obama beat Sen. John McCain (R-Ariz.) among Catholics by a 9-point margin, 54-45.

White, politically moderate Catholics will hold the cards this time, said University of Akron political science professor John Green. Although Latino Catholics tend to vote Democratic and highly observant Catholics generally side with the GOP, “middle-of-the-road Catholics are very much up for grabs,” Green said, noting this bloc is well-represented in battleground states such as Iowa, Ohio, Pennsylvania, Missouri and Wisconsin.

“It’s not clear to me today which way they would vote,” he added.

One factor is Republican fiscal policies, epitomized by Ryan’s $3.5 trillion fiscal 2013 budget, which would cut and revamp the Medicare program and reduce spending by $5.3 trillion over a decade, largely by slashing entitlements and government agencies. A Catholic, Ryan has cited his faith as a guiding principal in his budget plan.

The U.S. Conference of Catholic Bishops, which sided with GOP leaders on issues such as contraception mandates in the 2010 health care law, challenged the Ryan budget in a series of letters to Capitol Hill last week.

“We fear the pressure to cut vital programs that protect the lives and dignity of the poor and vulnerable will increase,” the bishops wrote to the chairman and ranking member of the House Ways and Means Committee. The bishops also voiced alarm over proposed cuts to Pell Grants, food and housing aid, low-income tax credits, and scholarship programs in letters to Senators and key Congressional committees.

Some Catholics have been more blunt. On the eve of a Ryan speech Thursday at Georgetown University, 88 professors on the Jesuit campus wrote to Ryan, challenging his “continuing misuse of Catholic teaching” to defend his budget.

Catholics United, a social justice group, held a rally at Georgetown to protest Ryan’s address and gathered 6,500 signatures on a petition calling on Catholic bishops to denounce the Ryan budget. The group has also been seeking face-to-face meetings with a handful of Catholic conservatives on Capitol Hill, including Reps. Steve Chabot (R-Ohio) and Mike Kelly (R-Pa.), to register concern about their votes in favor of the spending plan.

“We are focused on some of the Catholic lawmakers who are acting more in line with the tea party than they are with the teachings of Jesus Christ,” said James Salt, executive director of Catholics United.

Ryan acknowledged during the Georgetown appearance that “there can be differences among faithful Catholics on this.” The lawmaker, who has been mentioned as a possible Republican vice presidential nominee, earned a public rebuttal from the bishops conference after he said on Fox News last week that those voicing concerns “are not all the Catholic bishops.”

Since then, Ryan has labored to mend fences, penning an article this week in the National Catholic Register that spells out his argument that the national debt is the nation’s leading social problem and that the answer lies not with the federal bureaucracies but with families, communities, and state and local leaders.

It’s a message that some Catholics will embrace and others will reject, Green said.

“Many of those centrist Catholics would probably agree with Catholic intellectuals and with bishops that there are a lot of things wrong with Paul Ryan’s budget from a Catholic perspective,” he said. On the other hand, he added, those same Catholics would tend to side with the GOP on issues such as abortion and contraception.

As Ryan has learned, that presents challenges for both Democrats and Republicans.