Senate Democrats will place a $516,000 bet next week that Rep. Joe Donnelly can beat Republican Richard Mourdock in the race for Indiana's open seat.

A party official said Friday the Democratic Senatorial Campaign Committee is financing a new ad for Donnelly that will run statewide. 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 the tea party-backed Mourdock defeated Sen. Richard Lugar in May's Republican primary.

Attention on the Indiana race has grown as a handful of forces have changed the seat from a safe bet for a Republican to a heated battle. Republicans need to flip four seats to win control of the Senate, and Democrats have turned the Indiana seat into an unexpected challenge with consistent spending since May.

Conservative super PACs have hit back in recent months. The group Americans for Prosperity announced last month it was spending $700,000 on ads criticizing Donnelly for supporting the federal health care law, while the anti-tax Club for Growth dropped $800,000 earlier this month against Donnelly.

In total, Democrats tracking Indiana ad spending say Mourdock and his allies will have spent $2.9 million on air by the end of next week, while Donnelly and his supporters will have blasted out $3.3 million.

Groups associated with Senate Democrats, including Senate Majority Leader Harry Reid's American Majority super PAC, have been spending heavily in support of Donnelly.