Democratic Presidential candidate Senator Hillary Clinton greets the crowd at a rally in Philadelphia, April 21, 2008. [Photo: Reuters]

Backgrounder: A Look at Contests Won by Obama and Clinton

Senator Hillary Clinton of New York was declared by U.S. media winner of the Democratic presidential primary in Pennsylvania Tuesday.

CNN reported that with 12 percent of precincts reporting results, Clinton led rival Senator Barrack Obama of Illinois 54 percent to 46 percent.

Fox News, NBC and other media networks also declared Clinton winner based on exit polls.

With 158 delegates at stake, a Pennsylvania newspaper said that the state is "poised to become decisive in the Democratic presidential nomination."

This is the first time since 1976 that Pennsylvania will play a major role in a presidential nomination.

Latest pre-election polls of Pennsylvanians showed Clinton maintained a lead, but the margin has shrank to single digits since April.

Some facts of the primary appear to be favorable to Clinton.

Clinton may be helped by the fact that Pennsylvania conducts "closed" primaries, meaning that only registered Democrats will be allowed to vote.

She may also benefit from the fact that Pennsylvania have higher percentage of older, white and female population than that of America as a whole.

Past primaries show Clinton consistently win heavily in these groups.

Exit polls show she is winning among voters 65 and above by a 23 point margin, 61 to 38 percent.

She also received more support than Obama among white males, with 55 percent voting for her.

Clinton on Tuesday acknowledged her White House bid is on the line in Pennsylvania.

"Well, I have to win," Clinton told ABC. "I believe that's my task. And I'm going to do everything I can to win."

Meanwhile, calling Pennsylvania an "uphill climb," Obama had said he didn't predict a victory, saying "we feel good about where we are."

Pennsylvania will also hold a Republican primary, but Senator John McCain of Arizona has already won the 1,191 delegates needed to win the GOP presidential nomination, making the Pennsylvania Republican primary a merely "beauty contest".