Just 48 hours from now, we might very well know who the Democratic presidential nominee is. But we probably won't. On Tuesday, six weeks after their last showdown, Senators Hillary Clinton and Barack Obama will face off in the Pennsylvania primary, where 158 pledged delegates are at stake — the biggest prize left in the dwindling primary season.

Clinton, who once had as much as a 20-point lead over Obama in the state, has seen that lead dwindle down to six points or less in the weeks leading up to the contest, though she is still predicted to win the battle. But with the race so close and neither candidate likely to reach the 2,025 pledged delegates needed to win the nomination outright, what's really at stake in the primary?

For Clinton:
With just nine contests remaining after Pennsylvania, Clinton not only has to win the Keystone State bout, she has to do so convincingly in order to quell the rising calls for her to abandon the race in favor of party unity. Otherwise, she will be unable to close the delegate gap on Obama, who currently leads her by nearly 150 delegates, including pledged delegates and announced superdelegates, according to CNN's calculations. If Clinton can win by double digits, she will bolster her argument that she is the candidate who can beat presumptive Republican nominee Senator John McCain in the fall by taking states with big, diverse populations.

A big win could also give her momentum and help with fundraising going into the May 6 primaries in North Carolina — where Obama has a strong lead — and Indiana, where the candidates were about even at press time. And though he's made few major mistakes so far, a win could also prolong the race and increase the chances that Obama will fall victim to another gaffe like his now-infamous "bitter" comment about rural Pennsylvanians. Clinton has exploited the comment in ads and in Wednesday's televised debate.

If Clinton just squeaks by on Tuesday, the calls for her to drop out might increase. Pennsylvania is the ideal blue-collar sweet spot in her target demo, and a narrow victory would highlight Obama's momentum, as well as Clinton's inability to close the gap, as Democrats become more anxious to end the infighting among the candidates.

For Obama
If Obama can beat Clinton in the state where she has held a strong lead for months, it could be enough of an upset that the New York senator would be forced to throw in the towel. And while many superdelegates are sitting on the sidelines, waiting to see how the remaining contests shake out, an Obama win might be enough to get some off the fence and push him further into unbeatable status.

On Friday, The Associated Press reported that Obama is on track to finish the primary season just 100 votes shy of the 2,025 needed to win, and even in the days after the "bitter" snafu, he gained six more superdelegates to Clinton's one.

If Obama loses big, Clinton will likely use that as further proof of her contention that the Illinois senator is not able to win the big states and would not be as formidable an opponent against McCain as she would. But a narrow loss would keep Obama comfortably in the lead, since he would get a proportionate share of the delegates at stake. If he keeps it close and wins 53 percent of the remaining delegates in the rest of the primaries, the total share he's won in the races so far, the AP reported that Obama would finish the primary season at 1,945 delegates, just 80 short of the number needed to clinch the nomination. Clinton would need to win 65 percent of the remaining delegates just to draw even with Obama. She has only reached that number once so far: in Arkansas, where her husband, former President Bill Clinton, was governor for more than a decade.

Whether Pennsylvania proves to be a watershed for one of the candidates, or a wash, a recent USA Today story said that uncommitted superdelegates are not holding their breath for the results. Dozens of superdelegates who haven't picked a candidate yet say the Pennsylvania primary won't be the decisive factor in their choice, the paper reported. Instead, they are waiting until July 1, the deadline suggested by Democratic National Chairman Howard Dean. After Pennsylvania, 566 delegates are at stake in the remaining races, which will wrap up on June 3.