Spending in Indiana's tight U.S. Senate race topped $20 million this week, with new spending from the conservative Club for Growth, Senate Majority Leader Harry Reid's super PAC, and national Democrats and Republicans.

This week marks the most expensive yet in the closely watched race between Republican Richard Mourdock and Democratic U.S. Rep. Joe Donnelly. The candidates have been locked in a dead heat since the May primary, when the tea party-backed Mourdock unexpectedly ousted longtime GOP Sen. Richard Lugar.

The Club for Growth is spending $600,000 in Indiana this week, club spokesman Barney Keller said Wednesday. Reid's group is buying up $800,000 of air time, according to an aide for Mourdock's campaign and a Democratic source who are both tracking ad buys. Both spoke on condition of anonymity because the campaigns do not publicly track figures.

The Democratic source also said that groups supporting Mourdock are spending $2.1 million over the one-week period that began Tuesday, while $1.6 million is coming from Donnelly supporters. It's the most expensive one-week total yet in a race that has already blown away previous spending levels in any contest for a U.S. Senate seat in Indiana.

A new ad attacking Mourdock began airing this week from the super PAC backed by Reid, a Democratic Nevada senator.

"Washington's a mess, and Richard Mourdock would make it worse, saying 'It's time for confrontation,'" the narrator says over grainy footage of Mourdock jabbing his finger in the air. The ad then says Mourdock's first "confrontation" was threatening jobs by opposing the federal bailout of Chrysler.

The latest ad supporting Mourdock was released Wednesday and continues the strategy of tying Donnelly to Democratic leaders in Washington.

"Joe Donnelly's plan for the Senate? Keep Harry Reid and the liberal Democrats in charge," the narrator says as yellowed photos of Donnelly, Reid, President Barack Obama and House Minority Leader Nancy Pelosi flash on screen.

Keller, the Club for Growth spokesman, said the latest ad shows that Donnelly is "just another liberal" who supported Washington spending on wasteful projects, such as the Lobster Institute in Maine.

Donnelly has pushed back against that argument throughout his campaign, noting most recently that he voted with Republican Speaker John Boehner about 60 percent of the time.

Donnelly had trouble competing with Mourdock's fundraising from July through September, raising half of the $3 million Mourdock pulled in. But new campaign finance rules, pioneered by Mourdock supporter Jim Bopp, have diminished the role of candidate spending and helped turn Indiana's Senate race into a faceoff between Reid's super PAC and former President George W. Bush adviser Karl Rove's super PAC.

According to a Republican tally of spending booked through Monday, the top four players in Indiana's Senate race are Rove's Crossroads GPS, which has spent $3.8 million; the Mourdock campaign, with $2.1 million; the Democratic Senatorial Campaign Committee, with $2.1 million; and Reid's Majority PAC, which has spent $1.9 million.