(CNN) -- The war for Pennsylvania's Democratic delegate votes will be more like a series of battles.
Sen. Barack Obama greets diners Tuesday at Pamela's Diner in Pittsburgh, Pennsylvania.
Sen. Hillary Clinton's presidential hopes may rest on the state's more than 4 million Democratic voters as she faces off Tuesday with Sen. Barack Obama of Illinois.
The senator from New York trails Obama by 144 delegates, according to a CNN tally, but a win Tuesday could build momentum for her. The polls close at 8 p.m. ET.
CNN's latest "poll of polls" shows Clinton ahead by about 9 percentage points, but the state's diversity could prove tricky.
With 158 delegates at stake, the sprawling state runs from the Midwest to the Eastern Seaboard, with its two biggest metropolises -- Pittsburgh and Philadelphia -- separated by about 300 miles.
If Obama is to upset Clinton, it will begin in the Philadelphia-anchored southeast corner of the state, where about 30 percent of Pennsylvanians resides. Read John King's analysis of how the vote may play out »
Philadelphia is home to about 12 percent of the state's residents, and more than half of the voters there are African-American.
Obama will need not only to win the votes by a large margin, he will need voters there to come out in droves.
Obama also may have an edge in Philadelphia's affluent suburbs, namely Bucks and Montgomery counties. Watch a breakdown of Pennsylvania »
Both counties began the year in the GOP column, but Obama-driven voter registration has helped flip the counties from red to blue. See the votes county by county
Chester and Delaware, also in southeastern Pennsylvania, are more conservative counties, but they too have some affluent Democrats who will be key to Tuesday's race.
Clinton has roots in the eastern portion of the state -- between Allentown and Scranton -- and the white, blue-collar voters seem to be in her corner.
The same holds true for the western reaches of the state, from Erie to Pittsburgh, where white, blue-collar voters and Catholics are expected to cast ballots for the former first lady.
If the race comes down to the wire, the winner's fate could hinge on the northern and south-central reaches of the state, known as "the T."
Voters in the largely rural area generally swing conservative, and there are many white, rural Democrats that Clinton could mobilize if she needs them.
If it comes down to voters in "the T," Obama and Clinton can bank on a long night of vote-counting as election officials tally the ballots in dozens of rural counties in north and south-central Pennsylvania. E-mail to a friend
CNN's John King contributed to this report.