Sen. Claire McCaskill (D-Mo.) is out with the third TV ad of her re-election campaign, again going after the "special interests" that dominate Washington.

The ad, called "Back Again," claims that McCaskill is being attacked by "secret money" flowing from "big oil and insurance companies." These special interests want to end Medicare, cut student loans and continue to receive their own special tax subsidies, according to the ad.

"What Missourians need to know is that these Washington special interests aren't just trying to silence Claire; they're spending millions to distort Claire's record because they know the other candidates in this race would be a rubber stamp for their special interest agenda, whether it's protecting tax breaks for Big Oil, ending Medicare as we know it or privatizing student loans," said McCaskill's campaign manager, Adrianne Marsh.

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The excessive influence of special interests has been a theme of McCaskill's re-election campaign. Her first TV ad hammered on a similar message. So far, conservative outside groups have spent at least $3.45 million going after McCaskill.

In a recent editorial, The New York Times noted that McCaskill is facing an "uphill fight."

"Republican interest groups are outspending Ms. McCaskill and other Missouri Democrats by a 7-to-1 ratio; Ms. McCaskill herself is being outspent by 3 to 1," the paper wrote. "Though she has raised nearly $10 million, the amount could be dwarfed by the unlimited money at the disposal of Republican-oriented groups."

At least three Republicans -- Missouri Rep. Todd Akin, former state Treasurer Sarah Steelman and St. Louis businessman John Brunner -- are running for the GOP nod to challenge McCaskill in the general election.