Mitt Romney leads Rick Santorum 42 to 37 percent, according to a survey by Public Policy Polling. It’s the first time Santorum has lagged in the state since he won the Iowa caucuses in January and the latest sign that momentum in Pa. favors Romney.

Ron Paul takes 9 percent, with just 6 percent for Newt Gingrich.

Wrote the pollster:

Pennsylvania Republicans are expressing major doubts about Santorum’s viability both in the primary and the general election. Only 36% of GOP voters think Santorum has a realistic chance at the nomination to 54% who believe he does not. And when it comes to matching up against Barack Obama in the fall only 24% of Republicans think Santorum would provide their best chance for a victory while 49% think that designation belongs to Romney.

Indeed, 39 percent of Pennsylvania Republicans think Santorum should drop out of the race; 52 percent disagree.

“The momentum in Pennsylvania is moving completely against Rick Santorum,” said Dean Debnam, President of Public Policy Polling. “Mitt Romney has a great chance to deliver a final crushing blow to his campaign on April 24th.”

Several recent polls (Franklin & Marshall, Mercyhurst and Quinnipiac) have showed Santorum slipping in the state he represented in Washington for 16 years. It’s a net gain of 17 point for Romney since PPP’s poll in early March.

Pennsylvania is a must-win for Santorum, who already faces calls to exit the race. Indeed, he seemed to draw a line in the sand to reporters Wednesday night.

“We are going to win Pennsylvania, and we need to win Pennsylvania,” he acknowledged.

That echoes the sentiment of friend and political ally. Reports The Hill: “Sen. Jake Corman, a longtime friend of Santorum and his family, said if it appeared Santorum wasn’t going to win the state, the former senator could drop his campaign.”

PPP surveyed 403 likely Republican primary voters on April 4th via automated telephone interviews. The margin of error for the survey is +/-4.9%.