ST. PAUL, Minn. — Shortly after a priest's opening prayer and a screening of a short film on John McCain's faith, Sen. Sam Brownback stepped to the microphone and didn't waste words.

"Just to get to the whole meat of the matter, the Catholic vote is a swing vote," the Kansas lawmaker and Catholic convert said at a Catholic reception during this week's Republican National Convention.

"It is a critical vote in swing states," he said. "It is a vote we can win — but only if we work to win it."

Catholics are shaping up to be the battleground religious vote of 2008. Recent polls show McCain and Democrat Barack Obama neck and neck among white Catholics — a better indicator of swing voters because Hispanic Catholics lean Democratic. With an estimated 47 million U.S. Catholic voters, the stakes are huge.

Obama and McCain want to energize Catholics who line up with them ideologically. But the real prize is the increasing number of Catholics who don't identify with either major party.

The largest bloc of Catholic voters — 41 percent — identify as independents, up 11 percentage points from 2004, according to February polling for Georgetown University's Center for Applied Research in the Apostolate.

Neither presidential candidate lines up precisely with the breadth of Catholic teaching, but Catholic organizers for McCain and Obama are making the case that their man comes closest.

Former Oklahoma Gov. Frank Keating, co-chair of the National Catholics for McCain Committee, said in an interview that the McCain campaign is staging a "very aggressive" Catholic get-out-the-vote effort, dispatching surrogates to mobilize lay people at parishes and speak before anti-abortion groups and Catholic fraternal organizations.

In St. Paul, the independent Catholic Working Group, which works for Republican causes, invited Catholic McCain backers to three events: a Mass and reception at the Cathedral of St. Paul, a panel discussion on judicial philosophy and the forum at a hotel where Brownback and other Republican figures lauded McCain and lashed out at Obama.

One Catholic McCain supporter, Rep. Chris Smith of New Jersey, spoke almost exclusively about abortion at various events this week, hammering home the claim that Obama would be "the abortion president."

Brownback highlighted McCain's stances against abortion rights and gay marriage. He lauded McCain running mate Sarah Palin, whose star turn this week has energized conservative Catholics and evangelicals alike.

But Brownback also challenged the notion that Democrats are more in line with Catholic social justice concerns, suggesting that McCain's opposition to torture and support of comprehensive immigration reform provide an opening.

"I am not conceding the social ground," said Brownback, a former presidential candidate. "We are a pro-life and whole-life party."

Getting out the Catholic vote was clearly on Brownback's mind.

"It is no gimme vote," he said. "This is one you've got to dig in and work on a parish-by-parish basis, get the list, identify people that'll get out and vote and then get them out to vote."

Brownback did not explain what he meant by "get the list." In 2004, the Bush-Cheney campaign urged people to obtain church directories for voter mobilization, attracting criticism from some clergy.

Brian Hart, a Brownback spokesman, said the senator was "talking about identifying active parishioners who can both develop a list of other like-minded people in the parish as well as capitalize at the local level on the existing Catholics for McCain contact list."

A McCain campaign spokesman declined to say whether parish directories were in the campaign's plans.

Last week in Denver, the Obama campaign argued that his policies on the economy, environment and poverty fit the Catholic pursuit of the common good. They said his policies would reduce the number of abortions more than the Republicans would.

Obama's campaign has targeted Catholics likely to agree: young Catholics, social justice Catholics and women's religious communities. But it also has the endorsement of Douglas Kmiec, a constitutional scholar and former Reagan administration official who just published a pro-Obama book called "Can a Catholic Support Him?"

One unknown in the race: the voice of U.S. Catholic bishops. Denver Archbishop Charles Chaput has said Democratic vice presidential nominee Joe Biden, a Catholic supporter of abortion rights, should refrain from receiving Communion.

And several U.S. bishops have rebuked Democratic House Speaker Nancy Pelosi for misstating Catholic teaching on when life begins.

Former U.S. Ambassador to the Vatican Jim Nicholson, appearing at the same forum as Brownback this week, said more bishops need to speak out about core Catholic issues.

"And we need to help them," Nicholson said. "We need to give them cover, give them solidarity, because it can get very lonely for them."

But it's still rare for bishops to directly criticize politicians. Instead, Catholic dioceses nationwide have begun to distribute "Forming Consciences for Faithful Citizenship," an issue-based road map for Catholic voters.