A Democratic interest group that has been targeting Republican Senators opposed to the stimulus plan has now invested in a new set of ads, honing in on three centrist, GOP lawmakers, looking to bolster support for the plan.

"It's critical that the Senate pass President Obama's jobs and economic recovery bill right away," the ad from Americans United for Change argues. The group represents a large coalition of liberal interest and organized labor groups.

The radio ads target Sen. Arlen Specter (R) in Pennsylvania, as well as Maine Sens. Susan CollinsSusan Margaret CollinsThis week: House kicks off public phase of impeachment inquiry Senate panel clears controversial Trump court pick Republicans warn election results are 'wake-up call' for Trump MORE (R) and Olympia Snowe (R).

"Thankfully, Pennsylvania Sen. Arlen Specter is providing the leadership we need to get the job done," the Pennsylvania ad says. "Sen. Specter has worked with President Obama to reach an agreement on a plan that has the support from a broad range of groups."

A version of the commercial is also running in Nebraska, praising Democratic Sen. Ben Nelson.

The ads take a different tack from the group's previous ads on the stimulus, which sought to target a broader universe of Republican centrists in the Senate.

Listen to the Pennsylvania ad here.