/*Clinton*/ has led polls in the state, and any win by her could fuel questions about why /*Obama*/ hasn't been able to sew up the nomination, despite having more money, having won more states and having a lead in the popular vote and pledged delegates, according to ABC News' delegate scorecard.
However, Democrats say, a small victory by Clinton today won't be enough to turn momentum in her favor.
"Hillary Clinton needs a clear and convincing victory today in Pennsylvania if she wants to continue on in this nominating process," said Tad Devine, a Democratic strategist not affiliated with either candidate.
'Clear and Convincing Victory'
It remains to be seen, however, what will constitute a large enough margin of victory for Clinton. Devine argues the New York senator must win the Keystone State over Obama by double digits, and dig into Obama's delegate lead.
"If she wins by 10 points or more, it will be viewed as a clear and convincing victory, but if it's closer than that, it will be less than a clear and convincing victory," Devine said.
Speaking to reporters in Conshohocken, Pa., today, Clinton rejected that common argument, saying "a win is a win."
"But maybe I'm old fashioned about that. But you run a very competitive race at a considerable financial disadvantage. I think maybe the question ought to be why can't he close the deal?" she said, "Why can't he win a state like this one, if that is the way it turns out?"
As a sign of her optimism about her chances in Pennsylvania, Clinton is scheduled to hold an election night celebration at a hotel in Philadelphia tonight while Obama is slated to hold a rally with singer John Mellencamp in Indiana -- one of many states, including North Carolina, West Virginia, Oregon and Kentucky, holding the next-up primaries May 6.
Older, Blue-Collar Workers Key for Clinton
During the course of the Pennsylvania primary battle, the candidates went bowling and drank beer, with Clinton going one step further and downing shots of whiskey and appearing more comfortable at times than her opponent.
Clinton's edge in past primaries among blue-collar workers, those without a college degree, and older voters could propel her to another primary victory tonight, while Obama's campaign hopes his hold on younger voters and high interest in this primary from a record 218,000 newly registered first-time voters could erode some of her support.
Clinton Eyes Pa. Demographics
"The state of Pennsylvania, on its face, given the nature of its voting population -- older, white, Catholic, working class -- for the most part, these are all demographic groups that Hillary Clinton has done well with throughout the course of this nominating process," Devine said.
Preliminary exit poll results indicate that nearly six in 10 Pennsylvania voters are women; under 50 percent of Pennsylvania Democratic voters have a college degree; three in 10 Democratic voters are Catholic; and almost half are liberals.
"To win this decisive, one candidate or the other, either Clinton or Obama, needs to break into the other candidate's base," Devine said.
If Obama wins today or beats Clinton in states where she has a strong base of support, Devine said, the race could shift in the Illinois senator's favor.
"If it doesn't, I think the race will continue the way it has since Super Tuesday, really since Iowa and New Hampshire, just more of a see-saw than anything else, but one where Obama has been able to hold an advantage," Devine said.
Race To The End
Over a pancake breakfast at Pamela's Diner with his wife, Michelle, and owner of the Pittsburgh Steelers Dan Rooney, Obama said he doesn't expect the Democratic race to end until the last primary votes in June.
"I have come to the conclusion that this race will continue until the last primary or caucus vote is cast," Obama said. "And that's not far away."
Throughout the nominating process, Obama has done better than Clinton among young voters, urban voters, black voters and Democrats with college degrees.
Pennsylvania's system of allocating delegates may provide Obama with an advantage because some districts are worth more than others, based on how heavy the Democratic vote was in that district in the 2004 presidential and 2006 gubernatorial general elections.
This quirk in the Pennsylvania Democratic Party's delegate allocation system might allow Obama to make up delegate ground against Clinton, who is expected to win the state overall, because of his strong support in sections of the state that have a history of producing heavy Democratic turnout.
As the polls opened in the Keystone State this morning, both candidates got some last-minute jabs in on network and cable television interviews.
"I think there's a big burden on Sen. Obama tomorrow to prove that he can win a big state, because he hasn't really up until now," Clinton said in a pretaped interview with ABC's Chris Cuomo that aired on today's "Good Morning America."
Clinton also decried Obama's fundraising advantage going into the state's primary. Obama outspent Clinton on TV in Pennsylvania by 2-1.
"I know very well that I'm in a real fight here," Clinton said. "Every time I turn around there are posters at bus stops and train trips and he's got an enormous cash advantage."
Obama had $42 million cash on hand in primary money by the end of March, according to his latest Federal Election Commission report. In comparison, Clinton had $9 million, but was carrying $10 million of debt.
Obama sought to manage expectations for any Clinton victory.
"Sen. Clinton started off with a big lead here. She had a 20-point lead," Obama told ABC's Robin Roberts on today's "Good Morning America." "But we feel good about how we've chipped away at that lead."
A Quinnipiac University poll released Monday found Clinton with a seven-point edge on Obama in Pennsylvania. In mid-February, Clinton boasted a 14-point lead over Obama in Pennsylvania.
Obama managed to erode Clinton's lead in early polls despite garnering headlines when he said economic realities made people in the state "bitter" and characterized them as clinging to guns and religion. He later said he could have worded his thoughts better.
Negative TV Ads
The bitter Democratic battle turned even uglier in the lead up to the Pennsylvania primary, with the candidates launching negative ads and robotic calls against one another.
On Monday, Clinton launched an ad in Pennsylvania that questioned whether Obama is up to the task of being president in a time of crisis.
"Who do you think has what it takes?" the narrator asks over images of Osama bin Laden, headlines about the stock market crash of 1929, long gas lines from the 1970s oil-shocks and images of the Cold War, Hurricane Katrina and U.S. soldiers.
The ad quotes President Truman's famous line: "If you can't stand the heat, get out of the kitchen" to cast Obama as complaining about last week's ABC News presidential debate.
The Obama campaign accused Clinton of using "scare tactics" in the ad. The Illinois senator's latest ad says: "Sen. Clinton has internalized a lot of the strategies, the tactics that have made Washington such a miserable place."
Defending his campaign's harsher tone, Obama told ABC News' Robin Roberts today: "You've always got to measure if somebody throws an elbow at you, and after three or four times of gettin' elbows in the ribs, you know, at what point do you sort of say, 'OK, you know, we, we, we've gotta put a stop to that?'" Obama said.
Both campaigns launched automated calls to Pennsylvania voters in a last-minute effort to question the credibility of their opponent, with Obama questioning Clinton's record on gun control and Clinton questioning Obama's record on energy.
Making headlines as voters go to the polls, former President Clinton told a Philadelphia radio station Monday that the Obama campaign took his Jesse Jackson comment and "twisted it for political purposes" and accused Obama's campaign of "playing the race card on me."
At the end of the interview, Clinton turned to an associate and said, "I don't think I should take any s-- from anybody on that, do you?"
Today Bill Clinton verbally attacked a reporter who asked him about the comments.
"No, no, no, that's not what I said," Clinton told an NBC reporter. "You always follow me around and play these little games. And I am not going to play your games today. This is a day about election day, go back and see what the question was and what my answer was. You have mischaracterized it just to get a another cheap story to divert the American people from the real urgent issues before us, and I choose not to play your game today."
Taking on the GOP
Pennsylvania voters today choose between two bruised candidates after a grueling and nasty election fight.
Many Pennsylvania voters acknowledged the negative tone in the campaign, according to prelimiary exit poll results. However Clinton is bearing the blame -- two-thirds of Pennsylvania voters in preliminary exit poll results say Clinton attacked Obama unfairly; fewer, but still about half, also say Obama unfairly attacked Clinton.
While some party officials are wringing their hands about the bitter tone of the Democratic campaign, worried it will turn off independents and undecided voters and damage their chances against Sen. John McCain, R-Ariz., in the fall, others say despite the tone, voters mobilized to overturn eight years of the Bush administration and will rally behind either candidate if they win the nomination.
"A majority of Americans believe this country is headed off in the wrong direction," Devine said. "As long as [those] issues tend to dominate the electoral landscape, Democrats will have a real advantage."