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HARRISBURG, Penn. -- Three weeks before the Pennsylvania Republican primary, a state poll shows Mitt Romney leading Rick Santorum by five percentage points. Santorum, who trails Romney by hundreds of delegates, has acknowledged that Pennsylvania is a must-win for his campaign. Santorum represented the state in Congress from 1991-2007. Here's the breakdown from the Public Policy Polling survey: Mitt Romney: 42 Rick Santorum: 37 Ron Paul: 9 Newt Gingrich: 6 Last month, Romney polled at just 25 percent to Santorum's 43 percent in Pennsylvania, a positive swing of 17 percentage points for Romney. "The momentum in Pennsylvania is moving completely against Rick Santorum," said Dean Debnam, president of Public Policy Polling, a Democratic firm. The survey of 402 likely Republican voters in Pennsylvania has a margin of error of plus or minus 4.9 percentage points. More popular Yahoo! News stories: • What Romney can learn...