The results of next week's balloting in Texas, Ohio, Vermont and Rhode Island could persuade Clinton to concede - effectively crowning Obama as the presumptive nominee.
If not, the focus of the campaign will shift mainly to Pennsylvania for the seven weeks until the state's April 22 primary - giving it an unexpected and potentially pivotal role in a historic presidential campaign.
This year, Democrats will nominate either the first woman for president or the first black. Such opportunities don't come along often.
For the past 20 years, as other states moved their primaries and caucuses earlier in the campaign season, Pennsylvania's Democratic faithful have simply rubber-stamped candidates who already had won enough delegates to clinch the nomination: Sen. John Kerry in 2004, then-Vice President Al Gore in 2000, former President Bill Clinton in 1996 and 1992, and then-Massachusetts Gov. Michael Dukakis in 1988.
Before that, the primaries generated occasional excitement.
In 1968, Pennsylvania was among the states that backed then-Sen. Eugene J. McCarthy, a leading critic of the Vietnam War who lost the nomination to then-Vice President Hubert Humphrey at a Chicago convention marred by clashes between anti-war protesters and police. In 1976, former Georgia Gov. Jimmy Carter's victory in the Pennsylvania primary cleared the way for him to win the nomination that led to his one term as president.
The current primary race is vaguely reminiscent of the 1984 contest among former Vice President Walter Mondale, then-Sen. Gary Hart of Colorado and the Rev. Jesse Jackson for the nomination to contest then-President Ronald Reagan's bid for re-election.
Mondale was the establishment candidate at the time, as Clinton is this year, while Hart was the political upstart with new ideas, a role Obama plays, but Jackson was a wild card. Mondale won the primary by prevailing in western Pennsylvania and running strong enough elsewhere to blunt Jackson's victory in delegate-rich Philadelphia and Hart's support in the Philadelphia suburbs and central Pennsylvania.
At the 1984 national convention, a key to Mondale winning the nomination was near-unanimous support from hundreds of elected officials and party leaders who were "superdelegates," a then-new category that freed them from any obligation to support a particular candidate. Superdelegates also are considered crucial in this year's race, and Clinton is backed by a majority of those who have publicly endorsed a candidate.
Supporters of Obama and Clinton say the unusual nature of this year's race defies comparisons to any previous contest.
"We're in uncharted waters - nationally and locally," said Marcel Groen, the Montgomery County Democratic chairman and a superdelegate who has endorsed Clinton.
"One thing I've learned about politics is anything can happen," said Mark Singel, a former lieutenant governor and former state party chairman who is supporting Obama.
Gov. Ed Rendell, Clinton's most visible backer in the state, noted that whether the Obama-Clinton race reaches Pennsylvania hinges on next week's balloting.
"We're not there yet. We'll know March 5," he said. "We still could wind up being irrelevant."
--- Peter Jackson is the Capitol correspondent for The Associated Press in Harrisburg. He can be reached at pjackson(at)ap.org.
