A great look from Los Angeles Times reporter Janet Hook at the competing factions in both parties and how they affect the VP race:

That's the minefield Barack Obama and John McCain are tiptoeing across as they choose their vice presidential nominees. The goal of each: to find a running mate who adds appeal to the ticket -- while steering around the candidates who would leave key groups angry and ready to abandon the party.

The tension is most pronounced among Republicans, a reflection of long-standing fault lines between the party's social conservatives and the faction that focuses more on economic issues.

Conservative evangelicals, in particular, have a history of animosity toward McCain, the presumed GOP presidential nominee. Looking for a sign of commitment to their causes, many of them have urged McCain to choose Huckabee, a longtime pastor and former Arkansas governor with solid antiabortion credentials. Some are actively campaigning against Romney, who as Massachusetts governor had a mixed record on abortion and gay rights.

At issue for McCain is how much muscle social conservatives will put into helping his campaign. "The question is not whether social conservatives will hold their noses and vote for McCain; it's will they knock on doors and plant yard signs," said Gary Glenn, president of the American Family Assn. of Michigan.

For Obama, the presumed Democratic presidential nominee, one key question is whether he will pick a running mate who might make up for his lack of foreign policy experience. That has led some to point to former Georgia Sen. Nunn, a military expert. But Nunn is suspect among gay rights activists, because in Congress he opposed measures to expand gay rights.

Another consideration for Obama is whether to reach out to frustrated supporters of his former rival, Sen. Hillary Rodham Clinton (D-N.Y.), by putting a woman or even Clinton herself on the ticket. Further complicating his decision, Obama has heard complaints from women's advocates about possible No. 2 contenders -- such as Gov. Tim Kaine of Virginia and Sens. Evan Bayh of Indiana and Joseph R. Biden Jr. of Delaware -- whom they do not see as strong supporters of abortion rights.

Neither candidate will choose a vice president that would make one faction "abandon" the party. Since picking someone from the other party could very well do that, most talk of unity tickets is just talk.