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.

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.

Even though Senator Biden comes to the Democratic ticket with decades of political relationships and personal history in electoral battleground states that Senator Obama cannot match, the new team’s advisers acknowledge that Mr. Biden is not a cure-all for the political challenges facing Mr. Obama, of Illinois.

Senator Obama’s request is likely to cause consternation among party officials, who have struggled to maintain some authority over the primary calendar.