Indiana has quickly become a major battleground in the race for control of the U.S. Senate, with national Republicans and Democrats forking over more cash this week to tea party favorite Richard Mourdock and Democrat Joe Donnelly.

Republicans, who must gain four seats to take control of the Senate, extended a run of ads for Mourdock this week. The Democratic Senatorial Campaign Committee, meanwhile, is spending $439,000 in coming days on Donnelly's behalf.

A Democratic source who tracks ad spending confirmed the new buys on background because campaigns will not openly talk about how much they are spending. The Mourdock campaign has spent another $300,000 on Indiana's airwaves, bringing the total spent so far to more than $8 million.

Polls have shown Mourdock and Donnelly statistically tied since the general election began in May, after Mourdock defeated longtime U.S. Sen. Richard Lugar.

While Democratic groups went on the air early in Indiana, Republicans have only recently made it to the state's airwaves as they look to defend what would have been a safe seat had Lugar won the primary.

Mourdock and Donnelly have recently used car analogies to trade jabs. Mourdock's latest spot features a car veering to the left, off the road, and plowing through mud.

"Donnelly follows Obama and Pelosi, going the wrong way," the narrator says.

The Donnelly camp spent $223,000 on a similar spot which began airing last week. In it, Donnelly stands in the middle of a two-lane country road while an actor portraying Mourdock drives up and yells "Hey, Donnelly! It's my way or the highway!"

In both cases, the campaigns stick by their strategies of trying to paint their opponent as too extreme. Democrats have pushed that angle longer, but Mourdock's campaign recently settled on a fairly consistent message.