The Senate voted down a measure last week that would strip the Environmental Protection Agency of its authority to regulate greenhouse gases (or to regulate anything on the basis of climate change prevention), but the liberal interest group Americans United for Change is hitting a few senators who voted in favor of Alaska Sen. Lisa Murkoski's bill.

Versions of the same ad will air in the home states of Republican Sens. Scott Brown (MA), Richard Burr (NC), and Chuck Grassley (IA). The group says is spending $400,000 on the campaign.