Rep. Joe Donnelly (D-Ind.) has maintained a narrow lead over Indiana state Treasurer Richard Mourdock (R) in their hard-fought Senate race, according to a poll from Donnelly's campaign.

Donnelly leads Mourdock by 40 to 38 percent with 8 percent of voters backing Libertarian candidate Andrew Horning, according to the poll.

Publicly released partisan polls tend to be a bit more favorable to their candidates than other public polling. The only recent public poll, from the conservative Rasmussen Reports, showed Mourdock holding a 5-point lead.

Outside groups are spending heavily on the race, a sign both sides see it as highly competitive. Mourdock has struggled to woo centrist Republicans and independents since he beat longtime Sen. Dick Lugar (R-Ind.) in the primary. He is running well behind Mitt Romney in the GOP-leaning state, and is now featuring an ad touting Romney's endorsement.

If Donnelly can win, that would make it much easier for Democrats to hold on to the Senate, where they are currently defending more seats. Republicans need to net four seats to retake control if President Obama is reelected.

The poll of 603 likely voters was conducted from Oct. 17-21 by the Democratic Global Strategy Group and has a margin of error of plus or minus 4 percent.