A big win there eight years ago was the lifeline she desperately needed to extend the Democratic race into May. This time around, she believes a big win would effectively end the 2016 contest in April.
For the Republicans, Pennsylvania is largely a beauty contest, since party rules mean the delegates aren't bound to support the primary winner. Yet the spread could have a huge impact on Donald Trump's path to 1,237 votes on the first ballot of the GOP convention.
Pennsylvania is one of five contests on the final Tuesday of April, a night the two front-runners hope to use as a major statement a week after both delivered big wins in New York.
Rhode Island, Connecticut, Maryland and Delaware also have primaries — and opportunities to win delegates. But Pennsylvania is the marquee contest — because of its traditional role as the gateway to May in presidential politics, and because of the particular circumstances of this year's campaign.
Here are five questions we'll get answered Tuesday:
Who will win Democrats' argument over who is more committed to helping blue collar workers, who blame bad trade deals and indifferent politicians for disappearing manufacturing jobs?
There's no better stage than Erie, Pittsburgh, Scranton, Reading and Allentown.
Will Sanders be able to stop Clinton's delegate momentum by racking up votes in the white, rural areas she won against Barack Obama in 2008?
He'll need a big victory to prove he can win in another big, industrial state.
As front-runner, the stakes for Trump are obvious: keep winning and make a statement to those unbound delegates. But what about Cruz and Kasich?
Kasich got a few delegates in New York but has been mostly a non factor with the exception of his home state Ohio win. He keeps saying the map is moving to favorable territory; he needs to prove he's about more than hoping for a fifth ballot miracle in Cleveland. For Cruz, the temptation will be to skip ahead to May and Indiana. But if Trump steams out of April with momentum that includes a giant Pennsylvania win, that "stop Trump" Indiana firewall Cruz is betting on might not materialize.
Can Cruz and Kasich stand in the way of Trump's scramble for delegates and force an open convention?
Pennsylvania's rules leave most of its GOP delegates as free agents for now, so the candidates view Tuesday's primary results as key to their efforts to court delegate loyalty later.
Who of the final three Republicans is best suited to stretch the Electoral College map in November?
They'll make their cases in the critical, moderate Philadelphia suburbs, and in rural areas dotted with small Christian churches and filled with more conservative views on issues likes guns and abortion. It's a big, complicated state -- 283 miles across, 160 miles north to south -- a perfect laboratory for presidential politics.