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If tensions between Hillary Rodham Clinton and Barack Obama were on the rise in Pennsylvania last week, this weekend saw the pot boil over, as the two exchanged some of the most heated rhetoric of the campaign thus far over controversial comments made by Obama at a San Francisco fundraiser.

As Obama publicly scolded Clinton for her attacks on Sunday, Pennsylvania Sen. Bob Casey appeared on CNN to defend Obama, whom he has endorsed, against charges of elitism. Casey hits that same message in a new TV spot (subscription) released today, reaching out to blue-collar Keystone State voters by portraying Obama as a compassionate person who understands the challenges facing Pennsylvania.

Casey begins by acknowledging that Pennsylvanians really are "struggling" with economic hardships. "It has to change; but it won't until we change Washington," he argues. "I've seen [Obama] stand up to the lobbyists and special interests." Casey goes on to take a veiled swipe at Clinton, saying Obama is "tired" of "political games and division," and he subtly attempts to turn Obama's "bitter" comments into a strength for the candidate by pointing out that "Obama knows Pennsylvania's hurting."

It remains to be seen how voters in Pennsylvania, Indiana, North Carolina and other states yet to vote in the Democratic primary race will respond to the "bitter" remarks. But before this weekend's flap, Obama was closing the gap with Clinton in Pennsylvania, cutting her once-sizable lead down to just a few points.