“The biggest problem for Santorum is there’s a three-week lull, no way to break the media narrative that Romney has this sewn up and a continued race will only hurt the nominee,” Republican strategist Matt Mackowiak told The Hill on Monday. “Santorum’s desperately trying to survive until May… when does the pressure become too much?”

Gibson said many of the attacks Romney has leveled against Santorum have even more resonance in Pennsylvania: His endorsement of centrist former Republican Sen. Arlen Specter (Pa.) over now-Sen. Pat Toomey (R-Pa.) in their 2004 primary and his move from Pennsylvania to the Washington, D.C. suburbs with his family. Santorum’s “going Washington” was a major attack line from Sen. Bob Casey (D-Pa.) when he beat Santorum in 2006.

Another factor: Some of Toomey’s Tea Party backers remain angry at Santorum for his 2004 endorsement of Specter. Toomey himself has not endorsed Santorum, but recently praised Romney as a “conservative who will govern as a conservative.”