Rep. Joe Donnelly (D-Ind.) holds a narrow lead over Indiana state Treasurer Richard Mourdock (R), according to a new independent poll.

Donnelly leads Mourdock by 40 to 38 percent in the poll, conducted for Howey Politics Indiana by DePauw University. That narrow edge, with a relatively high number of undecided voters, mirrors recent polls released by Donnelly and other Democrats.

Both sides have been spending heavily on a race some once thought would be an easy hold for the GOP, even after Mourdock beat popular longtime Sen. Dick Lugar (R-Ind.) in an April primary. Mourdock is winning just six in 10 Republicans who backed Lugar in the primary, a concerning sign for his campaign. The Hill has long rated the race a "toss-up."

Donnelly and Democrats have sought to paint Mourdock as a hard-line Tea Party candidate and frame Donnelly as a centrist, while Mourdock has sought to try to make the race in the Republican-leaning state a simple choice between a Democrat and a Republican.

The poll of 800 likely voters was conducted on Sept. 19-20 and Sept. 23 and has a margin of error of plus or minus 3.5 percent.