ABC News' Michael Falcone reports:

In an attempt to eject Rick Santorum from the presidential race by force, Mitt Romney's campaign will unveil a nearly $2 million television ad buy on the Pennsylvania airwaves beginning on Monday.

According to sources tracking TV ad spending the buy, totaling $1.9 million, is widespread, hitting media markets from Philadelphia to Erie between April 9 and April 22 - two days before the state's April 24 primary.

(The Pittsburgh Tribune-Review's Salena Zito reported that the buy could grow even larger - eventually ballooning to nearly $3 million.)

Rick Santorum has already made clear that "we have to win" Pennsylvania and Romney predicted this week that Santorum, who served the state in the House and Senate, is "obviously going to do well in Pennsylvania." A loss in the Keystone State would be exactly the knock-out blow the Romney campaign is seeking to finally shut the door on the GOP primary.

But officials with the Santorum campaign say they do not plan to let the Romney 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."

With speculation running high that Santorum may quit the race before the primary, his campaign team circulated a tentative schedule of events in Pennsylvania from Tuesday through Friday of next week. Gidley said many more campaign stops would be added in the coming days. Santorum's wife, Karen, and daughter, Elizabeth, also plan to crisscross the state independently. (Neither Santorum nor Romney have any public events this weekend.)

The pro-Romney super PAC, Restore Our Future, has also purchased air time in Pennsylvania ahead of the primary as well as in New York, Connecticut, Delaware and Rhode Island - the other states that are also holding nominating contests on April 24.

Gidley said when the campaign does purchase air time in Pennsylvania they will use it to run a hard-hitting commercial that portrays Romney and Obama as the same person. It first aired in Wisconsin in advance of the primary there.

He said Romney's investment in the state "shouldn't be a surprise to anyone."

"This is Mitt Romney's pattern," Gidley said. "He comes in and carpet bombs his opponents in every state with massive ad buys."