HARRISBURG — Democratic U.S. Senate candidate Joe Sestak is airing his first TV attack ad in his bid to unseat five-term incumbent Arlen Specter of Pennsylvania.

The ad aired Wednesday follows Sestak's core message that party voters can't trust the Republican-turned-Democrat.

The May 18 Democratic primary is closely contested.

Specter is a centrist who is endorsed by President Obama.

In the Sestak ad, video footage shows Specter saying "my change in party will enable me to be re-elected." An unseen announcer follows with "Arlen Specter switched parties to save one job, his."

It also shows footage of Specter being endorsed in 2004 by then-President George W. Bush.