WASHINGTON (AP) - Pennsylvania was a must-win state for Hillary Rodham Clinton, the former Democratic presidential front-runner now looking to survive.
     
Should she lose, it would be a fatal blow to her candidacy - a defeat so unexpected that it would sap her arguments to remain in the race. It would be the dawn of a Democratic era minus the Clintons as the dominant force.
     
Clinton has been in must-win territory already this year - in New Hampshire and then Ohio - and both times she survived.
     
Another big victory could give her new energy in the face of calls for her to drop from the race in order to clear front-runner Barack Obama's path to the nomination. It would enable her to argue she's had victories in most of the country's largest states, give her a chance at overtaking Obama's lead in the popular vote and boost her prospects heading into the final stretch of the campaign.
     
There are seven states left to vote, and the candidates appear likely to split the spoils. Clinton is favored in West Virginia and Kentucky, while Obama is expected to take North Carolina, Oregon and South Dakota. Two states - Indiana and Montana - are competitive.
     
But Clinton has a tough reality to overcome to be nominated no matter what happens - Obama is practically assured to end the race with a lead in pledged delegates.

Even in Pennsylvania, a Clinton victory was never bound to net her much in the delegate count since urban districts where Obama is strong are rewarded more delegates than rural areas where Clinton is likely to do well.
     
So winning the nomination will require Clinton to get unpledged superdelegates to look past the lead held by the candidate with a shot at becoming the first black president. Many of those superdelegates are elected officials who have constituents to answer to.
     
No matter what happens in the rest of the race, the odds are stacked high against her winning the nomination. But Clinton has proven that she won't back down when she still has a shot, and a Pennsylvania win would give that to her.
     
Bill Clinton has already begun making a new argument for her candidacy - it's the Democratic Party rules that have kept her from becoming the nominee, he told a reporter on the eve of the vote.
     
The party rules split delegate support instead of allowing the winner of each state to take all the vote, as they do in the general election and in several states on the Republican side.
     
"We don't have a nominee here because the Democrats chose a system that prevents that result," the former president told The Washington Post.

Nedra Pickler covers the Democratic presidential campaign for The Associated Press.
     
(Copyright 2008 by The Associated Press.  All Rights Reserved.)