Outside groups have been spending heavily in the race, with Karl Rove's American Crossroads running television ads depicting Wilson as an independent voice and environmental groups in the state knocking her votes removing regulations on oil companies. But the environmental groups seem to be gaining traction; 46 percent of those who say they recently learned new information about Wilson said they had a less favorable view, versus 32 percent who said they changed their opinion in favor of the Republican.

Still, there's some encouraging news for Wilson. Despite the dropping favorability numbers and spending by environmental groups, her four-point deficit is actually a marginal improvement over a Public Policy Polling survey released in April that showed Heinrich with a 48-43 percent lead.

This post was corrected at 4:30 p.m. to reflect that pro-Wilson ads were aired by American Crossroads, not Crossroads GPS.