Mitt Romney trails Rick Santorum by just 2 percentage points in Santorum's home state of Pennsylvania, a must-win for the former senator, according to a new poll from Franklin & Marshall College.

Santorum leads Romney by 30 percent to 28, within the poll's margin of error. That's a huge swing from a month ago, when the pollster found him leading Romney 45 percent to 16 percent.

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It is difficult to see the rationale for Santorum continuing his campaign after the state's April 24 primary if he doesn't win his home state, which is also a major delegate prize. Few other states that vote in April are friendly territory for him, and if Romney wins in Santorum's home state the whispers from top Republicans that Santorum should drop out will likely increase.

The poll also found a wide-open field for the GOP race to take on Sen. Bob CaseyRobert (Bob) Patrick CaseyDems target small cluster of states in battle for House Overnight Health Care — Presented by Purdue Pharma — Trump officials move to require drug prices in TV ads | 4,000 more people lose Medicaid in Arkansas | New top official for Medicaid Election Countdown: Cruz, O'Rourke fight at pivotal point | Ryan hitting the trail for vulnerable Republicans | Poll shows Biden leading Dem 2020 field | Arizona Senate debate tonight MORE Jr. (D-Pa.): Eighty-one percent of registered Republicans are undecided, showing there has been little focus on the race.

The poll of 505 registered Republican voters was conducted from March 20-25 and has a margin of error of plus or minus 4.2 percentage points.