John Baer at the Daily News gets into one of the mysteries of the Senate campaign -- where the Sestak camp got that clip of Arlen Specter saying, creepily, "My change in party will enable me to be re-elected" in their attack ad last week.

Unfortunatley, no one's telling so far ("Specter's folks don't know where it's from; Sestak's camp won't say, but the grin and apparent smugness sure won't help Specter," Baer writes.) What's clear is the fight between "a veteran brawler [and] a brawling veteran" is a really rough one, the columnist says:

No surprise this is nasty.

The state's wily, longest-serving senator always plays hardball. And Sestak, despite a publicly genteel manner, didn't get to command an aircraft-carrier battle group in wartime by smiling and playing beanbag.