Senator Hillary Clinton played down expectations before Tuesday's primary in Pennsylvania, telling supporters that a "win is a win," no matter how slim the margin. But with 90 percent of precincts reporting at press time, Clinton was on track to score the kind of decisive double-digit victory she needed, leading rival Senator Barack Obama 55 percent to 45 percent.

Up for grabs in the primary were 158 delegates, and with the win, Clinton can marginally chip away at Obama's overall lead in pledged delegates, which stood at 1,648 to 1,504, according to CNN estimates before the Tuesday (April 22) contest.

Wearing a bright-green jacket, black shirt and black pants, and with daughter Chelsea and former President Bill Clinton behind her, Clinton took the stage at a Philadelphia rally shortly after 10:15 p.m. ET and gave a speech that wrapped up many of her campaign's biggest themes. "With two wars abroad and an economic crisis at home, you know the stakes are high and the challenges are great," she said. "But you also know the possibilities. Those possibilities are endless if we roll up our sleeves and get to work with a president who is ready to lead on day one," she added, to a roar of "Yes she will!"

Hitting another stump story she's told often over the past weeks, Clinton said the Pennsylvania victory was particularly emotional for her because the state was the place where her grandfather worked in the lace mills as a boy and where her father attended college and played football for Penn State. "I only wish they could have lived to see this moment. Because in this election, I carry with me not just their dreams, but the dreams of people like them and like you all across our country. People who embrace hard work and opportunity, who never waver in the face of adversity, who stand in what you believe and never stop believing in the promise of America. I'm in this race to fight for you."

Commending Obama and his staff, after weeks of slashing attacks, Clinton struck a conciliatory note, saying, "We are, in many ways, all on this journey together to create an America that embraces every last one of us." After a nod to Obama's spending power, Clinton also made a frank plea for her supporters to go to her Web site and contribute to a campaign that is seriously in debt and lagging way behind Obama in fundraising. "We still have a lot of work ahead of us," Clinton concluded. "But if you're ready, I'm ready. I might stumble and I might get knocked down, but as long as you'll stand with me, I will always get right back up."

With neither candidate likely to gather the 2,025 delegates needed to win the nomination outright before the August convention in Denver, it is once again clear that the battle will be decided by the party's superdelegates, who might have to risk going against the will of the majority of voters, should the race stay tight. Needing the win to not only stay in the race but to also keep on-the-fence superdelegates on her side, Clinton's convincing margin of victory Tuesday night may be enough to keep her campaign viable — at least until the next set of primaries in two weeks.

The Pennsylvania vote put an end to a six-week gap between primaries, during which the candidates focused almost exclusively on Pennsylvania, transforming what had been a relatively civil battle for the nomination into an increasingly bitter fight marked by attack ads, accusatory robo-calls and hard-hitting mailings. According to CNN, Obama spent more than $8.5 million on TV spots in the state since the beginning of the year to Clinton's $3.6 million. In the most ominous ad to date, Clinton launched a spot over the weekend that for the first time in the Democratic race invoked the specter of terror mastermind Osama bin Laden and asked the question, "Who do you think has what it takes?"

In an interview with ABC News, Clinton termed the closed primary — in which only registered Democrats could vote — a race " I have to win," while Obama sought to tamp down expectations and said he expected the results to be close but to fall in Clinton's favor in a state he called an "uphill climb." Obama has relied on the support of many independent or crossover Republican voters in previous primaries and caucuses but could not tap into that well of support in Pennsylvania.

A loss by Clinton, who is trailing Obama in both the popular vote and delegate count, would have surely increased the calls for her to drop out of the increasingly bruising race, which many party insiders and supporters fear could hurt the eventual Democratic candidate in the general election in November against presumptive Republican nominee John McCain. At one point, Clinton was leading Obama by nearly 20 points in Pennsylvania, but that margin was whittled down to single digits in the week leading up to Tuesday's vote. The win means that Clinton will continue to stay in the race and likely continue to claim that her wins in big states with large blue-collar populations are proof that she has what it takes to beat McCain in the fall, despite Obama's delegate and vote lead.

Not only was Clinton helped by the closed primary system, but she also likely got a boost from the fact that Pennsylvania has the second-oldest population in the country and voters older than 40 have tended to favor her so far by a margin of 54 percent to 40 percent, according to a recent Quinnipiac poll. Like a number of primaries this year, officials expected record turnout for the primary in a state that has not seen this much attention in a presidential race in recent memory and which saw 300,000 new voters registering as Democrats. According to some estimates, turnout could be more than 25 percent above the showing during the 2004 primary, with numbers that could look more like general-election totals than early nominating ones.

With music from John Mellencamp playing in the background (the Indiana rocker had performed for the Evansville, Indiana, audience earlier in the evening), Obama's concession speech in the next state to hold a primary opened with a congratulations to Clinton and a pledge that the thousands of new voters his campaign helped register will lead the Democrats to victory in November.

"These Americans cast their ballots for the same reason you came here tonight," Obama said. "For the same reason that millions of Americans have gone door-to-door and given whatever small amounts to this campaign. For the same reason that we began this journey just a few hundred miles from this spot, on a cold February morning in Springfield: because we believe the challenges we face are bigger than the smallness of our politics and we know that this election is our chance to change it."

Like Clinton, Obama stuck to some of the same campaign themes he's reiterated time and again. And, like Clinton, he avoided naming his opponent, but instead alluded to his desire to pump fresh blood into Washington to avoid repeating the mistakes of the past, exploiting his front-runner status by taking on McCain instead of Clinton. "Not this time. Not now," Obama said before enumerating a litany of complaints against the long-serving Republican senator on issues from the war in Iraq to tax cuts. "We already know what we're getting out of the other party's nominee. John McCain has offered this country a lifetime of service, and we respect that. But what he's not offering is any meaningful change from the policies of George W. Bush."

Challenging the audience to vote for what he called real change, Obama ended his speech by saying that if his followers are willing to "shed our cynicism and our doubts and our fears, if we're willing to believe in what's possible again, then I believe we won't just win this primary election. We won't just win here in Indiana. We won't just win this election in November. We will change this country. We will change the world. ... That's our task; that's our job. Let's get to work."

Next up are the Indiana and North Carolina primaries on May 6. Obama holds a commanding lead in North Carolina, and Clinton has campaigned steadily in Indiana, where the two are essentially in a dead heat. But if she loses the battle for the Hoosier State, several advisers would urge her to quit the race, according to The New York Times.

Get informed! Head to Choose or Lose for nonstop coverage of the 2008 presidential election, including everything from the latest news on the candidates to on-the-ground multimedia reports from our 51 citizen journalists, MTV and MySpace's Presidential Dialogues, and much more.