This new poll shows a slight increase in Donnelly's lead when compared to the last internal poll from his campaign, which gave him just a 1-percentage-point lead over Mourdock in August.

But the two are likely to remain locked in a tight battle, as the state looks to be going for Mitt Romney in the general election, though President Obama managed to take the traditionally red state by a slight margin in 2008. Both parties are investing funds there, and the National Republican Senatorial Committee reportedly just bought airtime in the state.

The poll was conducted among 800 likely voters from Sept. 10-12 and has a margin of error of 3.5 percentage points.