Arizona Sen. John McCain remains mostly unchallenged on the Republican ballot.
Only registered Democrats can vote for their party's candidates in the primary. At stake are Pennsylvania's 158 delegates to this summer's Democratic national convention - the biggest bloc of delegates still to be awarded.
Clinton is scheduled to make a major campaign swing through Pennsylvania this week. The stops are set to kick off with a policy address at the University of Pennsylvania on Monday, followed by stops in the Philadelphia suburb of Blue Bell and then Uniontown in western Pennsylvania.
Later in the week, former President Bill Clinton is slated to attend campaign events in Pottstown, Reading, Harrisburg, and State College on Thursday.
Obama is hoping to find new recruits in Pennsylvania to help him overcome Clinton's solid lead - 12 points in one poll taken last weekend.
Since last fall's election, statewide Democratic enrollment has swelled by more than 111,000 - an increase of about 3 percent in less than six months.