Senator Barack Obama (D-IL) speaks to a crowd of supporters at the Peterson Events Center in Pittsburgh, Pennsylvania, April 21, 2008.

Voters in Pennsylvania are casting their ballots Tuesday in a primary critical to the Democratic presidential nomination race.

For Democratic contenders Senator Barack Obama of Illinois and Hillary Clinton of New York, the key of the primary isn't so much who wins, but by how much.

Clinton: "I must win"

First of all, analysts agree Pennsylvania is a must-win for Clinton if she is to cut into Obama's lead in the overall delegate count and the popular vote and win the support of superdelegates.

The tiniest of wins for Obama would all but guarantee him the nomination, they said.

If the senator from Illinois wins, he'll claim that he has shown surprising strength in a state that is Clinton's demographic home turf, with many of the lower-income Democrats who have supported her in earlier primaries.

That kind of result would give Obama momentum heading toward the May 6 contests in Indiana and North Carolina, where a sweep would make a Clinton nomination feel all the more unrealistic.