Everybody says Mitt Romney has already won, so why is he carpet bombing Rick Santorum's home state?

Everybody says Mitt Romney has already won, so why is he carpet bombing Rick Santorum's home state?:

(Jim Young/Reuters)

Mitt Romney says it's over. Republican super delegates say it's over. But his campaign and his Super PAC are nonetheless planning another wave of television ad carpet bombing, this time in Rick Santorum's home state of Pennsylvania.

The $2.9 million advertising campaign will run in the Wilkes-Barre, Scranton, Erie, Altoona and Philadelphia media markets until the April 24 primary election.
Within a week, the ads will run in the Pittsburgh market. The Romney super PAC Restore Our Future is airing commercials on cable channels statewide.

Romney himself offered an insight into his campaign's thinking:

"Look, we all want to win our home states," Romney told the Tribune-Review. "I won Massachusetts, Gingrich won Georgia and I expect that Mr. Santorum will win Pennsylvania."

That statement combined with the fact that Romney is trying to win Pennsylvania suggests he's trying to pressure Rick Santorum to get out of the race before Pennsylvania in order to avoid the embarrassment of losing his home state, which recent poling suggests is entirely possible.
But whether Santorum stays in or out, doesn't Mitt Romney already have the nomination locked up? Why should he worry about Santorum? The answer is that Romney won't really have the nomination locked up until he hits his magic number. What he's got locked up is the ability to win the nomination; nobody can stop him but himself.
But between now and when he hits the magic number, Rick Santorum can embarrass Romney—and force him to work for the nomination. Romney is likely to lose many of the states that vote in May if Santorum stays in, and given that he's only at 41 percent in the latest Gallup tracking poll of the GOP primary, he still hasn't convinced a majority of Republicans that he'd be their best candidate.
Romney doesn't want that, so he's trying to force Santorum out by winning Pennsylvania. It's a strategy that might work, but if it fails—if Santorum stays in, and wins his home state despite Romney's efforts—it will only strengthen Santorum's ability to embarrass Romney.
6:43 AM PT: Mitt Romney is putting a temporary hold on negative ads due to the illness of Rick Santorum's daughter Bella:

Romney pulls negative ad from PA airwaves "until further notice" due to Santorum's daughter's hospitalization

— @michaelpfalcone via web

Romney, however, will still be on the air:

This is ad the Romney camp will be running in PA: http://t.co/... swapped in for an anti-Santorum spot

— @michaelpfalcone via web

It's telling that the only thing that can keep Romney from attacking is something as tragic as Santorum's daughter's illness.