After Sen. Barack Obama won handily in North Carolina's primary and rival Sen. Hillary Clinton scored a narrow victory in Indiana, many pundits called for Clinton to exit the race and bring the Democratic battle to a close. Mark Shields…

After weeks of heavy campaigning in Pennsylvania, Sens. Hillary Clinton and Barack Obama faced off in the Keystone state's Democratic primary Tuesday. Political analysts Mark Shields and David Brooks weigh the candidates' chances going into and coming out of Pennsylvania.

With Sens. Hillary Clinton and Barack Obama locked in a hard-fought -- and lengthy -- battle for delegates, the race for the Democratic nomination is pushing forward. Presidential historians discuss what past elections show about the impact of protracted primary…