Once a reliable Democratic voting bloc, Catholics have emerged as a pivotal swing vote in recent presidential races. Evenly divided in a poll over the summer, Catholics make up about a quarter of the national electorate and about a third in the pivotal battleground states of Michigan, Missouri, Ohio and Pennsylvania.

In an interview with NBC, Senator Biden tried to walk the line between the staunch abortion-rights advocates in his party and his own religious beliefs. While he said he did not often talk about his faith, he said of those who disagree with him: “They believe in their faith and they believe in human life.”

It is not uncommon for the Republican platform to diverge from the positions of the presidential candidate, who must appeal to independent voters for a general election. But given Senator McCain’s recent efforts to align himself more closely with party’s conservative orthodoxy, the differences are still striking.

At a lunch Friday in Minneapolis, two of John McCain’s top advisers — Charlie Black, a veteran political operative, and Dan Coats, a former senator from Indiana — were extolling Alaska Gov. Sarah Palin’s virtues to about 150 influential evangelicals as evidence of the senator’s ideological commitments.

Aides to Senator McCain said they had a team on the ground in Alaska now to look more thoroughly into Sarah Palin’s background. Sources said the team assigned to vet Ms. Palin in Alaska had not arrived there until Thursday, a day before Mr. McCain stunned the political world with his vice-presidential choice.

After the stunning announcement by Senator John McCain on Friday that he had chosen 44-year-old Alaskan Governor Sarah Palin as his running-mate, new information emerged over allegations that Ms. Palin tried to use her gubernatorial office to take revenge on her former brother-in-law.

Many of the social measures on the ballots are being pushed by evangelical groups that hope to force Senator John McCain of Arizona, the presumptive Republican presidential nominee, to pay closer attention to their agenda. The proposals that are likely to attract the most attention are same-sex marriage and abortion amendments, along with proposals to ban affirmative action.

Senator Barack Obama, the presumptive Democratic nominee, lost the Catholic vote badly to Senator Hillary Rodham Clinton, who, like Mr. Obama, is a supporter of abortion rights, during many state primaries. In Pennsylvania, Catholic voters preferred Mrs. Clinton to Mr. Obama by a 40-point margin.