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“Those organizations are going to be looking at winning other states, too,” Mr. Robertson said. “Massachusetts might be more winnable than Missouri, the Virginia race is going to require a lot of money. There are better bets for these groups to make than Missouri.”

The Akin campaign did receive some good news last week, when the Missouri Farm Bureau announced it wouldn’t withdraw its endorsement and bought nearly $20,000 in radio ads for Mr. Akin and Dave Spence, the Republican candidate for governor.

Mr. Akin also had enough funds to launch a new ad this week, attacking Mrs. McCaskill over her vote for the health care law. This month, he’s spent more than $100,000 on TV ads in the state’s major metropolitan areas of St. Louis and Kansas City, according to filings with the Federal Communications Commission.

But that pales in comparison to Mrs. McCaskill, who has spent nearly $600,000 running ads in St. Louis and Kansas City during the same time period. She also has reserved hundreds of television spots next month.

Mr. Akin doesn’t deny he’ll be heavily overspent, telling voters last week that they need to base their support on how each candidate has voted in Congress, not on what they see on TV.

“Pretty soon, there’s going to be $10 million worth of advertising saying Todd’s a terrible guy,” he said, speaking in Dexter, Mo. “The main thing you got to remember is keep your eyes on how people vote.”