Senate Democrats are betting $516,000 on congressman Joe Donnelly's chances of beating Republican Richard Mourdock in the race for Indiana's open seat.

A party official says the Democratic Senatorial Campaign Committee is financing an ad for Donnelly that will run statewide next week. The official wasn't authorized to speak publicly ahead of the ad's release and requested anonymity.

The battle for Indiana's Senate seat has been tight since tea party-backed state Treasurer Richard Mourdock defeated Senator Richard Lugar in May's Republican primary.

Democrats have spent heavily on the air throughout the summer trying to box in Mourdock. But conservative super PACs have hit back in recent months.

The group Americans for Prosperity has spent $700,000 on ads criticizing Donnelly while the anti-tax Club for Growth has spent $800,000.