Older, whiter and more female than the nation as a whole, Pennsylvania looks like Hillary Rodham Clinton country. Wealthier, better educated and more African-American than the rest of the state, Pennsylvania’s thickly settled southeast corner could belong to Barack Obama.

For six weeks, the two Democratic presidential rivals have courted their political bases and sought to carve up each other’s support with an increasingly tart tone. After both deluged the state with ads, crisscrossed it on buses, planes and trains, and had a parade of surrogates march through it, Clinton holds a slight lead.

Pennsylvania’s primary Tuesday has the potential of being decisive, or it could extend a campaign that has gone far longer than most ever imagined.

Clinton, the New York senator, is looking to validate her case that only she can win big state primaries. Obama, the Illinois senator who leads in delegates and in the national popular vote, wants to shut Clinton down so he can turn his attention to the presumed Republican nominee, John McCain.

A defeat for Clinton here could be devastating. One of her key backers, New Jersey Gov. Jon Corzine, said last week that a Pennsylvania loss for her would be “pretty much a door closer.”