The group American Rights at Work is targeting Sen. Arlen Specter (D-PA) with a new ad asking whether he'll stand "with Obama, Biden, and the working families of Pennsylvania, or with greedy CEOs and big business lobbyists" on Employee Free Choice.

"We hope Senator Specter will join the President and the majority of Congress who understand that if we truly wish to restore our middle class, workers must be able to bargain, not borrow their way to a better life," said Kimberly Freeman, Acting Executive Director of ARW.

The ad is among the first to target Specter on any issue since he switched parties last month, and by far the most explicit. In recent days, progressive groups have seemingly demonstrated a renewed willingness to target conservative Democrats. Earlier this spring a variety of campaigns aimed at pressuring House Blue Dogs and their Senate counterparts were scrapped (or all-but scrapped) after party leaders said the initiatives weren't helping.

Specter cosponsored the Employee Free Choice Act last Congress, but reneged that support this spring when, as a Republican, he faced a 2010 primary challenge from conservative Pat Toomey. Now, as a Democrat, he's supposedly working toward a compromise with the bill's lead sponsor, Sen. Tom Harkin (D-IA).