Clinton won this go-round. With 98 percent tallied late last night, she held a solid lead of 10 percentage points, 55-45, and network analysts believed that she would post a net gain of about 15 delegates.
But Clinton's victory - powered by strong support from her core base of white women, older voters and union members, weighted toward western and central Pennsylvania - did little to change a nationwide equation still in Obama's favor.
Obama's strength in the southeastern corner of the state offset Clinton's gains elsewhere and kept her margin in delegates small.
That means that the Illinois senator still has a solid lead nationwide in delegates - up by 132, according to MSNBC - and a half-million lead in popular votes cast, little changed from when this all began back in the wind and frost of mid-March.
Most experts see a growing quandary for the Democrats as they seek to rally behind a candidate to oppose GOP Sen. John McCain in the fall: They don't see how Clinton can win the nomination, yet they don't see what could make her drop out, especially with a Pennsylvania win under her belt.
"One thing about the Clintons is they know that every day matters and something big could happen any day," said Larry Sabato, the University of Virginia history professor and presidential analyst. "Their goal is to stay alive as long as possible."
"The bottom line is that she's won Pennsylvania, and she's going forward," NBC News pundit Tim Russert said on MSNBC last night, minutes after she was declared the winner.
Indeed, the candidates were already starting the next lap of their long-distance race. After crisscrossing Pennsylvania by planes, trains, and buses, Obama had already moved onto the next big battleground state of Indiana last night, before the votes here were even counted.
Speaking in Evansville, Ind., to an enthusiastic crowd that was warmed up by rock star John Mellencamp, Obama strived to put the best spin on the results in Pennsylvania, where he trailed in the earliest polls by nearly 20 percent.
"There were a lot of folks who didn't think we could make this a race when we started," Obama said. "They thought we were going to be blown out."
He stuck to his campaign message of the need to change America's political culture and said he is still the Democrat who can best do that.
"We rallied people of every age and race and background to the cause," Obama said, adding, "And it is those new voters who will lead our party to victory in November."
Clinton appeared misty-eyed and emotional as she came out with her ex-president husband before a throng of cheering supporters at the Bellevue's Grand Ballroom in Center City last night. "It's a long road to 1600 Pennsylvania Ave. and it runs right through the heart of Pennsylvania!" she said.
She continued to play up the themes that she believed carried her to victory: that she was best qualified to serve as commander in chief and fix the troubled economy. "And I thank you, Pennsylvania, for deciding I can be that president."
But the biggest obstacle for Clinton in the future contests may not so much be votes or delegates as money. Obama has some $42 million in the bank while the New York senator has at best $8 million, not enough to open many field offices across Indiana or hit every media market in North Carolina.
Last night in Philadelphia, Clinton made a public appeal to her backers to immediately go to HillaryClinton.com and make a donation to maintain her campaign's momentum.
Ironically, many of the leading pundits are already talking about the May 6 primary in Indiana - with its mix of rural voters and the industrialized areas around Lake Michigan - as a make-or-break battle in the same way they once described Pennsylvania.
Right now, most experts believe the race will focus more on Indiana - where polls have shown a tight race and a possible slight Clinton lead - than on North Carolina, which also votes on May 6. That's because Obama is an overwhelming favorite in the Tar Heel State, thanks to the state's large populations of blacks and educated high-tech workers who have broken his way in past primaries.
But Virginia analyst Sabato said last night that May 6 is not likely to force Clinton out of the race even if Obama can add Indiana to his predicted win in North Carolina.
That's because Clinton is all but assured of winning three more primaries, in West Virginia and Kentucky, thanks to her overwhelming support in Appalachia, and in Puerto Rico, where the Clintons are very popular.
Nathaniel Persily, a Columbia University law professor and political pundit, noted that Clinton will also continue to use her victories in big states, now including Pennsylvania, to make a case to a couple of hundred uncommitted and unelected party leaders called "superdelegates" that she should get the nomination.
One key issue that remains in play is what to do about Florida and Michigan, which were stripped of their delegates by the Democratic National Committee (for holding their primaries too early) but where Democrats still voted in January, with Clinton winning both contests. Her longshot quest for the nomination may hinge on winning a battle to seat those delegates.
"Think about the convention as a jury that decides on the merits of a case that the candidates will lay out," Persily said. "She has a set of arguments, and the remaining primaries will give her evidence why she should be the nominee."
The evidence that Clinton will be able to present from Pennsylvania will show that she would bring back to the Democrats a strong base of support among working-class voters, especially from blue-collar males in western Pennsylvania who voted for the GOP as "Reagan Democrats" in the 1980s and 1990s.
Exit polling by the television networks showed that Clinton won overwhelmingly in that western part of the state - especially in the rusty ex-steel towns and coal-mining communities that ring Pittsburgh, where she racked up 70 percent of the vote.
Another huge factor in her win was overwhelming support from voters over age 65 in the state with the nation's third-oldest population, behind only Florida and West Virginia.
But ironically, despite Clinton's victory, the lasting impression from the epic political battle here may be the ways that Pennsylvania has changed - especially in the Philadelphia suburbs, where registered Democrats are now a plurality for the first time.
And those suburbs gave surprisingly strong support to Obama - 62 percent, according to an NBC News exit poll - which is the main reason he dampened Clinton's margin of victory.
One thing that sometimes gets lost in all the punditry is the enthusiasm that both Democrats generated, especially here in Philadelphia. Voters turned out in record or near-record numbers for a presidential primary in many divisions.
First-time voter Rosanna Matos, 21, of North Philadelphia, pulled the lever for Clinton. "Her husband has the experience of being president," she explained. "It's a more recognizable name."
"I don't want the same old, same old," countered Jean E. Spriggs, 69, a retired schoolteacher from Rittenhouse Square who voted for Obama. "I'd like to see change. I'd like to see young people involved." *
Staff writers Kirsten Lindermayer, Regina Medina, William Bender and Dafney Tales contributed to this report.