But party rules don't bind delegates to the candidate they've committed to, and Clinton has been dropping the idea that she could poach some Obama delegates as the hard-fought nomination battle goes to the convention in Denver.
"Remember that pledged delegates in most states are not pledged," Clinton told the Daily News editorial board last week. "You know, there is no requirement that anybody vote for anybody. They're just like superdelegates."
While analysts think most pledged delegates are hard-core partisans who won't stray, City Controller Alan Butkovitz, who's running in the primary as a Clinton delegate, said this is a year in which anything could happen.
"What do [Obama] delegates do if Bill Clinton shows up at their house?" Butkovitz asked. "He's the best salesman there is. He's a very potent weapon who brings tremendous emotional power."
"I think that's why the Clintons haven't given up. They know they have lightning in a bottle," Butkovitz said. "You've got two and a half months after the primaries before the convention. If they're 80, 100 votes short, do they think they can pick up a delegate a day?"
Both campaigns will continue to try to woo superdelegates in cases where they see hope for movement.
With U.S. Sen. Bob Casey's endorsement of Obama last week, only seven of the 26 Pennsylvania superdelegates chosen so far - three more won't be named until a party confab in June - haven't publicly committed to a candidate.
But the 14 Clinton supporters and five Obama superdelegates are free to change their minds up until the moment they vote at the convention.
Are pledged delegates just as free?
A national party rule established in 1982 requires pledged delegates to "in all good conscience reflect the sentiments of those who elected them."
Party officials say that leaves wiggle room, and if delegates could be convinced Obama would lose in the general election, they would be free to defect, even though it's likely he'll finish the primaries with a lead in delegates and the popular vote.
Of course, it's just as possible Clinton delegates might switch to Obama to end the divisive battle for the nomination and unite the party.
In a contest this close, partisans will look for every edge, and Pennsylvania is a state in which Clinton has strong support among top party leaders.
Three of the state's superdelegates and 55 of its pledged delegates will be chosen in June at the convention of the Democratic State Committee, which is heavily influenced by Gov. Rendell and whose chairman is T.J. Rooney. Both are strong Clinton supporters.
Could party leaders stack the deck, handing Clinton the three superdelegates, and assigning Obama pledged delegates whose loyalty is questionable?
Absolutely not, says Rooney.
"This is not a situation where we can sneak in Clinton delegates," Rooney said. He noted that the lists of pledged delegates to be voted upon by the state committee are submitted to the campaigns for approval.
"The onus is on the candidates to ensure that the people they've slated stay true to the cause," Rooney said.
What about the three superdelegates to be chosen in June?
"A lot of people will make requests [to be superdelegates]," Rooney said. "We play things straight. And the fail-safe for anybody who's concerned the deck is stacked is that they have to be approved by members of the state committee, many of whom support Obama."
Clinton and her campaign officials have insisted they're not trying to talk Obama delegates into switching, and Rooney said he's heard no such talk in Pennsylvania.
But he acknowledged that if the race is still contested over the summer, it's hard to predict what might happen.
Ironically, one potential pledged delegate who admitted to the possibility of defecting is a Clinton supporter from Philadelphia.
J. Whyatt Mondesire, publisher of the Sunday Sun and president of the Philadelphia NAACP, is running in the primary as a pledged Clinton delegate.
He expects to vote for Clinton, but says that extreme circumstances could make him re-think his convention vote.
"If it got to a situation where the convention was breaking down we were going to hand the election to the Republicans, and 50 or 100 delegates were moving to Obama to seal the deal, I might join them," Mondesire said.
"Stopping the destruction of this country is paramount," Mondesire said. "We can't afford to lose."
One other footnote to Pennsylvania's delegate selection: It's often reported that the state will have 188 delegates to the convention, but the actual number is 187.
One of the state's superdelegates, union official and Obama supporter Anna Burger, changed her voter registration to Washington, D.C., earlier this year. The state is not allowed to replace her in the delegation.
You can see a detailed explanatinon of the delegate selection on the state Democratic Party's Web site: http://www.padems.com.