The decisions by Obama and McCain will be determined by a wide range of considerations, including personal chemistry and regional appeal. But while the candidates are looking at what qualities might help the ticket, they are hearing from various factions about potential disqualifying factors.
Many evangelical conservatives have bridled at reports indicating that Romney is a strong contender for the No. 2 Republican slot. Those reports have arisen frequently, because Romney's business background is seen as complementing McCain's resume, which is light on economics.
"We don't have any faith that he will be consistent on the moral issues we really care about," said Mike Fracassa, a vice president of Ohio Christian University, about Romney.
Last month, dozens of evangelical leaders met in Denver to talk politics. The group drafted a letter, which was hand-delivered to McCain, urging him to name Huckabee.
"Putting Gov. Huckabee on your ticket will immediately excite, mobilize and activate a key grass-roots constituency that is essential to your success," the letter said.
Janet Folger, who heads a conservative group called Faith 2 Action, was more blunt. "Mitt Romney would be a disaster," she said in an interview. "Mike Huckabee would be a home run."
Economic conservatives and tax-cut advocates see things differently. Pat Toomey, president of the anti-tax Club for Growth, has criticized Huckabee for raising taxes as governor of Arkansas.
"Huckabee would be a disaster," said Toomey. "It would thoroughly alienate economic conservatives."
Many of the GOP's business allies have no litmus test for McCain's running mate and take a more pragmatic view of the decision.
"I care more about winning than anything else," said one business lobbyist, who asked not to be named because his organization is neutral in the campaign. He said that former Pennsylvania Gov. Tom Ridge would be a big asset in an election in which his home state is crucial, but acknowledged that his support for abortion rights would make his nomination problematic.
"The right wing would go crazy," the lobbyist said.
One of the riskiest choices McCain could make would be to tap Sen. Joe Lieberman of Connecticut, an independent who in Congress is aligned with Democrats, but who is supporting McCain and has been hawkish on the Iraq war.
Grover Norquist, head of Americans for Tax Reform, called Lieberman "completely unacceptable," because he is liberal on most other issues.
