ABC News' Michael Falcone reports:

With Rick Santorum's young daughter, Bella, in the hospital, Mitt Romney is yanking a negative television ad from the Pennsylvania airwaves "until further notice," campaign officials said on Monday.

The ad, part of the Romney campaign's plan to blanket Pennsylvania media markets ahead of the state's April 24 primary, was originally meant to remind voters of Santorum's landslide 2006 Senate re-election loss

"We fired him as senator," the ad's narrator says. "Why promote him to president?"

Instead, the Romney team has swapped in a positive ad in its place.

"We have done this out of deference to Sen. Santorum's decision to suspend his campaign for personal family reasons," Andrea Saul, a spokeswoman for the former Massachusetts governor said in a statement. Saul said the campaign informed television stations to pull the ad Monday morning and that broadcasters would "comply with this request as soon as they are technically able."

Santorum's campaign announced on Friday that the former Pennsylvania senator's three-year-old daughter, Bella, had been hospitalized. She suffers from a condition known as Trisomy 18, a genetic disorder that is often fatal.

Santorum is not scheduled to hold any public campaign events until Tuesday, but this weekend his campaign said he would also be curtailing his private political schedule on Monday.

"Senator Santorum will not hold any campaign related events on Monday so that he and Karen can remain in the hospital with their daughter Bella," communications director Hogan Gidley said in a statement. "The entire Santorum family is incredibly grateful for the outpouring of prayers and support."

The Romney campaign had already purchased TV airtime in the Keystone State - a buy totaling $1.9 million and spanning media markets from Philadelphia to Erie between Monday and April 22, two days before the state's primary. Santorum has indicated that the April 24 contest is a must-win for him.

Over the weekend, officials with the Santorum campaign said they did not plan to let Romney's ad buy go unanswered.

"We'll make an ad buy, no question about that," Santorum's national communications director Hogan Gidley told ABC News on Saturday, noting that although the campaign has yet to purchase any airtime, he fully expects they will do so "soon."

This is the positive ad the Romney campaign will be running in Pennsylvania for now: