Santorum had said he had to win his home state to have a chance in the race with former Massachusetts Gov. Mitt Romney.
Since Santorum left, however, Romney is overwhelmingly favored to emerge from the Republican National Convention in Tampa, Fla., as the party's challenger to President Obama.
Pennsylvania, with its 72 delegates, joins Connecticut, New York, Delaware and Rhode Island in holding primaries Tuesday, the first primaries since Santorum announced April 10 he was suspending his campaign.
But as Romney effectively seizes the Republican nomination, his one-time rival's presence could linger in the tough months ahead, pundits said.
Observers note Santorum managed to expose Romney's weaknesses, such as an inability to excite voters and a moderate track record that could win over independents in November but left conservatives and evangelicals in the primaries cold, the Tampa Bay News reported.
Santorum emerged from a crowded field to take the Iowa caucuses and 10 other states, forcing Romney to cough up millions of dollars he had planned to spend to defend his positions and challenge Santorum. The campaign rhetoric between the two was sometimes vitriolic.
"The real question for Romney now is, can Republicans unite? Can he get enthusiasm out of the base?" Pennsylvania pollster Terry Madonna asked.
Pennsylvania is Santorum's home turf and he vowed to win it -- but Romney also promised to be competitive. Before he left, Santorum's once-double digit lead had dropped to the low single digits.
Now that Santorum has bowed out, the Romney campaign pulled television advertising from the Pennsylvania airwaves, CNN reported. Romney had purchased $2.2 million of time for two ads.
The Wall Street Journal reported, however, a super PAC supporting Romney will spend at least $290,000 in positive campaign spots in Pennsylvania, Delaware, New York and Rhode Island, which all hold their primaries on Tuesday, along with Connecticut.
Pennsylvania voters will be asked for photo identification at polling places but won't be barred from casting ballots if they can't produce any, state election officials said.
The statewide balloting is a test-run for the new law that, beginning with the Nov. 6 general election, will require voters to show a photo ID that meets state guidelines every time they vote, The Wilmington (Del.) New Journal reported.
Tuesday's exercise is to help educate voters and identify problems that can be eliminated before Election Day and the larger turnout that's expected.
Redistricting has been blamed for the re-election woes of Blue Dog Democrat Tim Holden, now considered the underdog in the primary race, Roll Call reported recently. About 80 percent of the redrawn district is new for Holden, who's also losing the battle of the bucks in northeastern Pennsylvania.
Holden was hit with a double whammy last week when the League of Conservation Voters announced a $230,000 TV ad buy targeting Holden and MoveOn.org endorsed Holden's opponent, attorney Matt Cartwright.
Cartwright has injected about $400,000 of his own cash into his campaign. The incumbent-targeting Campaign for Primary Accountability also pledged to spend $200,000 against Holden.
In a crime novel twist, a Pennsylvania state lawmaker running unopposed in the primary will become constitutionally ineligible to serve Tuesday when he is sentenced on a conviction for corruption, WITF-FM, Harrisburg, reported.
Former state House Speaker Bill DeWeese, a Democrat, campaigned for another term even though he gave a farewell address on the Pennsylvania House floor. But he won't be a felon -- at least, not technically -- until Tuesday when he is sentenced.
In February, DeWeese was convicted of five corruption charges, with a jury determining he used about $100,000 worth of state resources for campaigning. When DeWeese left the courtroom, he didn't apologize, saying, "I certainly feel I did nothing wrong."
He also didn't think his conviction barred him from campaigning even though the state Constitution bars felons from holding office.
DeWeese, who represents Greene County in southwestern Pennsylvania, could be sentenced anywhere from probation to six years in prison and he'll officially be a felon. If he doesn't resign, the House of Representatives will expel him.
Barring a successful legal challenge, DeWeese's name will be on the fall ballot. If he wins an appeal before then, DeWeese says he'll be back.