INDIANAPOLIS (AP) — Senate Republicans will jump into Indiana's pitched Senate battle this week, responding to a Democratic ad-buy with one of their own as they seek control of the Senate in November.

The National Republican Senate Committee will spend $650,000 on an ad that will run statewide on broadcast and cable for one week starting Tuesday. A Republican Party source tracking ad buys confirmed the new spot Sunday on condition of anonymity because it has not aired yet.

The NRSC buy tops Senate Democrats', who have spent $516,000 on a spot airing at the same time, and marks the committee's first foray on air in Indiana this cycle.

The air war in Indiana has quickly escalated as Democratic candidate Joe Donnelly has stayed neck and neck with Republican Richard Mourdock since May.

Mourdock's primary victory over Sen. Richard Lugar created the opening for Democrats in Indiana and has forced Republicans to divert resources to what would have been a safe seat for them if Lugar had won.

Republicans, who need to flip four seats to win control of the Senate, have seen their chances toughen with surprising comments by Rep. Todd Akin in Missouri and the pending retirement of Sen. Olympia Snowe, R-Maine.

Outside spending has a major role in Indiana's general election battle, much as it did in the primary race. Conservative groups Americans for Prosperity and the Club for Growth have spent a combined $1.5 million, and super political action committees associated with Senate Majority Leader Harry Reid and congressional Blue Dog Democrats have combined for roughly $1 million on Donnelly's behalf.

The NRSC buy brings the total spent buy Mourdock and his supporters to $3.5 million. Donnelly and his supporters have spent $3.3 million. The candidates' spending totals include super PAC buys.