Joe Sestak Hammers Arlen Specter With Ad Showing Bush, Palin

The Pennsylvania Democratic Senate primary fight between Rep. Joe Sestak and Sen. Arlen Specter (who became a Democrat last year after almost 30 years as a Republican) is going down to the wire.

According to the latest Muhlenberg College tracking poll (PDF), Specter leads Sestak 48 percent - 40 percent, with 11 percent either not sure or choosing someone else.

Now Sestak has unveiled a tough new ad portraying Specter as a political opportunist who only became a Democrat so he can win reelection. (See it at left.)

The spot shows Specter saying his party change will allow him to remain in office as well as footage of President George W. Bush effusively praising Specter. It also includes a shot of Specter with Sarah Palin.

"Arlen Specter switched parties to save one job: his, not yours," an announcer says.

Specter has the backing of the Democratic establishment, which sees him as the better candidate for the general election; as Politico reports, the chairman of the Pennsylvania Democratic Party this week suggested a Sestak victory in the primary could have "cataclysmic" consequences.

"If we want to keep this seat in Democratic hands, the only person capable of delivering that victory is Arlen Specter," said chairman T.J. Rooney.

CBS News chief political consultant Marc Ambinder reports that a senior political strategist consulted by the White House believes there is a 50/50 chance Specter will lose.

Democrats are nervous about the race: President Obama is expected to campaign for Specter in the coming days, as Ambinder notes, and Sestak has indicated the White House offered him a high-ranking administration job in exchange for dropping out of the race, prompting calls for an investigation from GOP Rep. Darrell Issa. 

The primary takes place on May 18th.