Your guide to the Pennsylvania primary

WASHINGTON (CNN) -- Pennsylvania voters will finally get to weigh in on the Democratic presidential race Tuesday, the first contest on the primary calendar in six weeks.

Sens. Hillary Clinton and Barack Obama will be competing for 158 delegates.

Obama currently leads Clinton 1,648 delegates to 1,504, CNN estimates. Neither candidate will pick up the 2,025 delegates needed to win the Democratic nomination before the convention, likey leaving Democratic superdelegates to decide the Democratic presidential nominee at the party convention in late August. Superdelegates are Democratic party or elected officials who can vote at the national convention. They choose their vote and are not required to commit to a particular candidate before the convention.

In the six weeks since the Mississippi primary, the Democratic race has taken on a particularly negative tone, with both candidates launching waves of robocalls, tough mailers and matching attack ads. Watch how the race has turned nasty »

The two candidates also spent millions on television advertising. Since the beginning of the year, the Obama camp has spent over $8.5 million in television spots in the state, while the Clinton camp spent $3.6 million.

Most political observers say Pennsylvania is a must-win for Clinton if she is to cut into Obama's lead in the overall delegate count and the popular vote and win the support of superdelegates.

Calls for Clinton to drop out of the race have increased from within the party as the candidates have stepped up attacks on each other. The increasingly pointed attacks have stoked fears among some Democratic leaders that the bruising primary battle could hurt Democratic chances in the general election. Watch what is at stake for Clinton »

Thus, not only must Clinton win, one of her biggest supporters in the state said, she must win convincingly if she is to stay in the race. "Well, she has to," Rep. Jack Murtha, D-Pennsylvania, told CNN. "That's all there is to it."

"If Clinton wins by more than 10 points, which was her margin in neighboring Ohio and New Jersey, her campaign will have new momentum and she will soldier on," said Bill Schneider, CNN senior political analyst. "If Clinton wins by single digits, we're in a political twilight zone. Nothing changes."

A CNN "poll of polls" of likely Pennsylvania Democratic primary voters calculated Monday shows Clinton leading Obama 51 percent to 42 percent with 7 percent unsure.

The poll of polls is an average of three polls conducted by Zogby, Suffolk University and Quinnipiac University conducted between April 18-20. No margin of error can be calculated for the poll of polls.

Clinton may be helped by the fact that Pennsylvania conducts "closed" primaries, meaning that only registered Democrats will be allowed to vote. Obama has benefited from the support of independent voters in previous primaries.

Clinton may also benefit from the fact that Pennsylvania has the second oldest population in the United States, behind only Florida. A Quinnipiac poll conducted April 18-20 has Clinton leading Obama 54 percent to 40 percent among voters 45 and over. Obama leads Clinton 57 percent to 41 percent among voters under the age of 45. The poll's margin of error was plus-or-minus 3.1 percentage points.

Obama is expected to do well in Philadelphia, which has a large African-American population, and the suburban counties surrounding Philadelphia, home to many upscale voters that in past primaries have tended to back the Illinois Democrat.

Clinton, on the other hand, is expected to do well in the more blue-collar city of Pittsburgh, located in western Pennsylvania, as well as the largely rural area in the middle of the state.

Pennsylvania will also hold a Republican primary, but Sen. John McCain has already won the 1,191 delegates needed to win the GOP presidential nomination, making the Pennsylvania Republican primary meaningless. E-mail to a friend

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