Finding a running mate tops the summer to-do lists of John McCain and Barack Obama, and each needs to decide what he wants as much as who he wants for vice president.
Are they out to offset their own perceived weaknesses, or reinforce an area of strength? Are they looking for someone from a particular region or demographic niche? How much should personal chemistry count in the equation? It's all about achieving balance.
The end of the Democratic primary season this past week touched off considerable debate about the relative merits of an Obama-Clinton "dream ticket." Obama could decide to pick vanquished rival Hillary Rodham Clinton on the strength of her campaigning skills, or steer clear of the political baggage she and her husband would bring. He could view adding her to the ticket as a way to reach out to women and unify the party, or see her as a divisive figure who might turn away general-election voters.
A similar balancing act will play out on the Republican side. McCain, 71, could try to use his running mate to bulk up the ticket's economic credentials, for example, or to inject some youthful vigor.
There are names afloat for just about any scenario, and Clinton's is surely one of the most buoyant.
"She is hardworking; she is tough; she is very smart," Obama said recently. "So I think she'd be on anybody's short list of the vice presidential candidates, but beyond that, I don't want to offer an opinion."
If Obama wants to amplify the Democratic ticket's sense of history by adding a woman, Clinton is not the only choice. Also on the list of potential candidates are Govs. Janet Napolitano of Arizona and Kathleen Sebelius of Kansas and Missouri Sen. Claire McCaskill.
Obama could pick a Clinton backer, such as Gov. Ted Strickland of Ohio or Gov. Ed Rendell of Pennsylvania, to attract her supporters. These governors offer another advantage; they know how to connect with people in the industrial Midwest, where Obama has struggled to address the economic anxieties of white, working-class voters. Former Sen. John Edwards of North Carolina also has working-class appeal, although he insists he does not want a second try at the vice presidency four years after he ran with John Kerry.
People are also worried about the Iraq war and national security matters, and if Obama wants that kind of experience on his ticket, he might pick former Sen. Sam Nunn of Georgia, a longtime Armed Services Committee chairman, or one of his former presidential rivals, New Mexico Gov. Bill Richardson, a former UN ambassador, or Sens. Joe Biden of Delaware or Chris Dodd of Connecticut.
Obama might seek an unconventional running mate, such as New York Mayor Mike Bloomberg, a Democrat-turned-Republican-turned-independent, or Nebraska Republican Sen. Chuck Hagel, a critic of the Iraq war. Both flirted with presidential bids last year.
Governors have an edge over other contenders; they have actually run governments, while Obama and McCain, both senators, lack executive experience. A governor can also be viewed as an outsider to a senator's Washington insider.
If Obama picks a governor from a competitive state _ such as Tim Kaine of Virginia, where Democrats have made gains in recent years _ he would have an outsider with executive experience who might help expand the electoral map.
"It's a very complex balancing act, kind of a Rubik's Cube, trying to find that combination that's going to get you to the White House," said Paul Light, a government professor at New York University.
Obama recently began looking in earnest, asking Caroline Kennedy, daughter of the late President John F. Kennedy; former Deputy Attorney General Eric Holder, and Jim Johnson, the former Fannie Mae CEO, to start the vice presidential vetting along with a tight circle of advisers.
For McCain, there are fewer Republican governors and fewer Republican lawmakers in Congress to include in the mix. Reagan administration hand A.B. Culvahouse is advising McCain on the process, according to Republicans close to the campaign.
McCain spent the Memorial Day holiday weekend vacationing in Arizona with four potential contenders: Charlie Crist, governor of Florida, a crucial state in November; one-time rival Mitt Romney, the former Massachusetts governor; Louisiana Gov. Bobby Jindal and Kansas Sen. Sam Brownback.
Jindal, 36, "violates the rule that you can't have someone half your age on the ticket with you," Light joked.
Besides Crist and Jindal, governors who might be considered include Minnesota's Tim Pawlenty, South Carolina's Mark Sanford and Alaska's Sarah Palin, or former Pennsylvania Gov. Tom Ridge.
Pawlenty, Sanford, and South Dakota Sen. John Thune all are well-liked by a range of party conservatives, a factor that McCain may give weight to given his sometimes rocky history with this core segment of the Republican base. Former presidential rival Mike Huckabee, the former Arkansas governor, is a favorite of Christian conservatives, as is Brownback.
On economic issues, Romney stands out as a venture capitalist and Olympics CEO. And his broad base of campaign contributors could help McCain's own bottom line. Other possibilities are former Texas Sen. Phil Gramm, a former Banking Committee chairman, and Ohio's Rob Portman, a former White House budget director and former congressman.
He might consider Connecticut Sen. Joe Lieberman, the 2000 Democratic running mate who switched parties to independent. Although Lieberman, a McCain booster, says he does not want to be on the ticket.
Outside-the-box contenders for McCain might be a former CEO such as Carly Fiorina, who ran Hewlett-Packard, former eBay CEO Meg Whitman or Fred Smith, FedEx's founder. All are involved in McCain's campaign.
For all the handicapping inside and outside the campaigns, in the end, voters still care most about the nominee. That is why Thomas Mann, analyst at the Brookings Institution think tank, scoffs at the idea that putting Clinton on the ticket would somehow deliver her constituency to Obama.
"There's no magic bullet way of using a running mate to deal with particular problems," Mann said. "People are going to vote on the basis of the presidential nominee, and there's not much that you can compensate with via the running mate."