Mourdock is also struggling in the "doughnut counties" surrounding Indianapolis, GOP pollster Matthews says. These counties are home to "a significant chunk of the state's Republican base," and are 20 points more Republican than Democratic. But the race is within the margin of error there, with Mourdock leading by just 4 points. Romney is leading in these counties by 18 points.
"Can Richard Mourdock win? Of course," Matthews writes in the Howey Politics Indiana newsletter. "Could he lose? Yes."
A slight gender gap was evident in the poll: Mourdock leads among men by 4 points, 42 percent to 38 percent, while Donnelly carries female voters by 6 points, 41 percent to 35 percent.
The Senate race is not the only open-seat contest on Indiana ballots this fall, but it is the only one currently competitive. Republican Rep. Mike Pence leads the gubernatorial race over folksy former state House Speaker John Gregg, 47 percent to 34 percent.
The poll was conducted Sept. 19-23, though no weekend interviews were conducted on the 22nd or 23rd. The two firms surveyed a total of 800 likely voters.