You can't say it any better than Dan Hirshhorn does here:

It’s the campaign ad everyone knew was coming—and it was only a matter of time.

The latest TV spot by Democrat Joe Sestak’s Senate campaign highlights former President George W. Bush’s support for Senator Arlen Specter during the 2004 campaign, as well Specter’s support for vice presidential candidate Sarah Palin in 2008. It also twice plays a clip of Specter saying that “my change in party will enable me to be reelected.”

Here's the video: