Despite Mitt Romney’s Tuesday night sweep, Rick Santorum isn’t ready to throw in the towel yet.

While Romney gave a rousing general-election speech Tuesday night, raking what he called “Barack Obama’s government-centered society,” he still hasn’t locked up the Republican nomination. And while Romney cruised to victory in Maryland, where Santorum didn’t campaign, and Washington, D.C., where Santorum wasn’t even on the ballot, he pulled out a tight win in Wisconsin, despite a 4–1 television spending advantage.

To finally push out Santorum, Romney will have to best the former senator in his home state of Pennsylvania, where voters have preexisting opinions of him that cannot be as easily changed by television commercials. Santorum emphasized this in his election-night remarks in a hotel ballroom north of Pittsburgh, telling his fellow Pennsylvanians, “You’ll be seeing the negative ads but you know me, you know how hard I work and how hard I believe.”

Saying the 15-month old Republican race was just now at “halftime,” since “Pennsylvania and half the other people in this country have yet to be heard,” Santorum vowed to go on, as his campaign banked on a win there in three weeks, in order to stay alive for what could be a big month in May in Texas and other delegate-rich states. Despite or perhaps because of Romney’s now overwhelming delegate lead, Santorum spoke darkly of the “party establishment and aristocracy” that he said shoved moderates “down our throats.”

Although it has been much bandied that Santorum will be hurt by his blowout loss in his U.S. Senate reelection bid in 2006, that was in a general election where moderates and swing voters abandoned him in droves. Conservative Republicans stayed loyal to Santorum in that race. They will be far less likely to believe the attacks on Santorum as a big spending, earmark-loving liberal that have been effective for Romney in other states.

A win on Santorum’s home turf would be the knock-out blow Romney has yet to deliver.