Even so, Rendell, a Clinton supporter, said a joint ticket led by either one "would be great."
History's lesson: They work
Wounds left by primary battles are one reason that no presidential candidate in a generation has chosen his top primary opponent to join him on the ticket. In 2000, George W. Bush tapped Halliburton CEO Dick Cheney, who had retired from politics eight years earlier. In 1992, Bill Clinton chose Tennessee Sen. Al Gore, who had cited family considerations in sitting out the primaries that year.
There are precedents, though — and history demonstrates that a team of rivals often prevails.
In 1960, Democrat John F. Kennedy selected the older, more experienced Texas Sen. Lyndon Johnson, who had hoped to claim the presidential nomination himself. Johnson, then the Senate majority leader, accepted the No. 2 spot, but relations between him and the Kennedy camp were never easy. Still, he delivered what JFK wanted, bolstering his strength among Southern conservatives generally and carrying Texas in particular.
In 1976, after a Republican nomination battle that went all the way to the national convention in Kansas City, Mo., President Ford and challenger Ronald Reagan didn't join forces. Ford didn't ask, Reagan told associates he wasn't interested — and Ford narrowly lost the election to Jimmy Carter.
Four years later, Reagan won the nomination and, after a flirtation with Ford, chose the elder Bush as his running mate.
Stuart Spencer, one of Reagan's chief campaign strategists, recalls discussing the vice presidential pick during the five-hour flight to the National Convention in Detroit from Los Angeles.
"Reagan spent the first 20 minutes dumping on George Bush," Spencer says. "He was very unhappy with George Bush and some of the attacks made on him in the primaries — 'voodoo economics' and that. I listened to the entire thing. He said, 'What do you think about it?' I said, 'I think you're going to pick George Bush.' "
Reagan needed to reach out to moderate Republicans and reassure voters concerned about his lack of Washington experience. Bush helped do both.
Too much change?
For Democrats this time, "whichever one of them ends up as the nominee, they have to make an appeal to the other person's voters," says John Podesta, a veteran Democratic operative who was president Clinton's chief of staff and now heads the Center for American Progress in Washington. While there are several ways to do that, he says, "one direct line is to offer the vice presidency to the person who comes in second."
At the moment, though, tensions between the two contenders continue to grow, and a down-to-the-wire nomination battle might not leave enough time for tempers to cool. "There's pride on the line," Johnston says. "After having split the vote 50-50, it'd be difficult for each to concede to the other."
A grinding month of campaigning leading up to the Pennsylvania primary on April 22 is likely to make things worse, he says. "The situation is set up for maximum nastiness, (and) the worry is the next few weeks will dry up whatever goodwill there is."
Some Democratic activists also are concerned that having both a woman and an African-American on the ticket could create unease among some voters accustomed to white men as president and vice president, making it harder to win the White House. "An Obama-Clinton or Clinton-Obama ticket is too big a bite for the American voter to take at one time," Madison says.
The traditional formula would call for either one to balance his or her ticket with a white man, perhaps one with a centrist mien and a swing-state address.
Spencer says strategists would need to "measure this issue with the public" — that is, take polls and hold focus groups — before settling on an Obama-Clinton or Clinton-Obama ticket.
"I can make the case it's too big of a change for them to accept all of a sudden," the Republican consultant says of the American electorate, while noting he's seen generational shifts in response to Obama's campaign this year that make the combination worth considering.
"It could be a powerhouse," Spencer says.