[cnn-photo-caption image= http://i.cdn.turner.com/cnn/2008/images/04/16/art.clinton.obama.jpg caption="Both Hillary Clinton and Barack Obama have establised bases according to exit polls."]
(CNN) - The Democratic race has been going on for four months. Is anything changing? The exit polls will tell us.
Each candidate has established a base. Hillary Clinton's includes women, seniors, blue-collar workers, Catholics, and Latinos.
Barack Obama's base includes African-Americans, young voters, affluent professionals, and Independents.
Tonight's exit polls will show whether either candidate is making inroads into the other candidate's base. If nobody's moving, we may be doomed to more weeks of trench warfare, with each side trying to rally higher turnout from its base.
How will we know things are changing?
If she wins Pennsylvania by more than 10 points, which was her margin in neighboring New Jersey and Ohio, we'll know the race is shifting in her favor.
If she wins by a very narrow margin, she's losing momentum.
And if Obama beats her in Pennsylvania? She will face powerful pressure to withdraw rather than further divide the party.