Washington, April 13: Hillary Clinton on Sunday battled to keep her White House hopes alive going into a tense 10-day stretch which could define the end game of her enthralling Democratic tussle with Barack Obama.

Anything less than a big win in the Pennsylvania primary on April 22 would severely dampen her hopes of taking the race to the end of the nominating calendar in June, in her long-odds bid to outdo Obama.

But remarks by Obama -- labelling working class voters as "bitter" -- could give Clinton the wedge she needs to pick up ground in the race, and she took off with the issue Saturday by suggesting her rival was condescending toward a large segment of US voters.

"Senator Obama's remarks are elitist and are out of touch. They are not reflective of the values and beliefs of Americans. Certainly not the Americans that I know," she said at an Indiana rally.

After six weeks of long distance sparring, Clinton and Obama will clash in a face-to-face debate in Philadelphia on Wednesday night.

Clinton, 60, trails Obama, 46, in total nominating victories, the popular vote and elected delegates going into the Pennsylvania contest, and Perception right now is crucial to her being able to continue," said Julian Zelizer, an elections analyst and history professor at Princeton University.

"The perception that she has the right to continue, the perception that she has the possibility of winning," Zelizer said.

Clinton meanwhile must also target the still-undecided "super-delegates" -- top party officials now crucial to deciding the nomination.

Pennsylvania should be exactly the kind of state where Clinton can triumph, with its traditional blue-collar Democrats, receptive to her populist economic message as fears mount of a serious economic recession.

But those advantages set up fierce expectations for Clinton, and failure to meet them with a convincing win -- of 10 points or more -- may be seen more as a defeat than a victory.

badly needs a convincing win to quell questions about why she is still in the race.