The Specter-Obama ad that began running yesterday is being funded by the Democratic Senatorial Campaign Committee, says Greg Sargent at the WashPost's Plum Line blog:

It's not surprising that national Dems would seek to protect incumbents. But the spending on Specter's ads suggests he's in real trouble. Also, Specter is an unusual case: He's been a Republican for most of his career, so it seems particularly awkward for him to be benefitting from national party money against a longtime Dem.