"Republican voters in the Philadelphia suburbs are more liberal on gays, guns and abortions than the Democrats in any part of the state with the exception of inner-city Philadelphia," he said.
A narrow victory may not be enough for Santorum, who could be under pressure on primary night to quit the race if Romney wins big in other liberal states like New York and Connecticut which hold primaries on the same night as Pennsylvania.
The polls that put Santorum ahead do not make him a strong home-state front-runner compared to his rivals' performances on their home turf.
Romney took an overwhelming 72 percent of the vote in the primary in Massachusetts, where he was governor. Former Speaker of the House of Representatives Newt Gingrich, running third in national polls, won his home state Georgia with 47 percent.
A Quinnipiac University poll on March 14 showed Santorum with 36 percent support in Pennsylvania. Romney was at 22 percent in the poll but he had trailed early in other large states, including Illinois and Florida, that he went on to win easily.