The issue of abortion may be key to McCain's choice.
The two contenders who support abortion rights -- former Pennsylvania Gov. Tom Ridge and Connecticut Sen. Joe Lieberman -- would be an enormous fight for McCain and would alienate a segment of his conservative base.
McCain advisers this past week have been meeting with social conservatives to gauge opposition to those picks, and they've been warned of a brewing revolt that could include a walkout at the Republican National Convention next week and a huge battle on the floor -- especially if the pick is the Independent Democrat Lieberman.
A recent ABC News/Washington Post poll found if John McCain were to pick a running mate who favors legal abortion, it could cost him votes, particularly in some core Republican groups.
Among current McCain supporters, 20 percent said they'd be less likely to vote for McCain if he picked a candidate for vice president who favors abortion rights.