McCaskill quickly sought to capitalize on the attack by citing it in her own fundraising plea.
The jostling over the latest ad by Crossroads GPS is proof that the national spotlight's shining on Missouri, where Republican- and Democratic-leaning groups are expected to spend millions of dollars on a Senate seat that could be pivotal to party control.
Crossroads GPS, which has ties to Republican strategist Karl Rove, said it is spending $991,000 to run a two-week advertisement on Missouri broadcast and cable outlets that criticizes McCaskill's support for the 2009 stimulus law signed by President Barack Obama. The ad contends the stimulus package resulted in wasteful spending on questionable projects.
The ad buy is almost twice as large as anything Crossroads GPS previously purchased in the race and raises its total spent in Missouri to $3.7 million during the past year.
"We're going to turn up the heat and keep up the pressure on President Obama and allies like Sen. McCaskill to try to fix those issues" related to federal spending and the nation's debt, said Nate Hodson, a spokesman for Crossroads GPS.
McCaskill's campaign responded with an email fundraising plea Wednesday, asking supporters to help her to counteract "outside attack groups" coordinated by Rove that "can now come after Claire with truckloads of money from shady secret donors" as a result of a U.S. Supreme Court decision on campaign finance laws.
Because it was established as a not-for-profit advocacy group instead of a political organization, Crossroads GPS does not have to disclose its donors.
During campaign events, McCaskill has been highlighting the undisclosed financing of groups running ads against her. At a rally earlier this month in Springfield, for example, McCaskill began by telling the crowd of about 100 people: "They're after me. Have you noticed?" But she said the millions spent on ads against her had been "a big waste of money" so far because they had done little to change her standing in public opinion polls.
"This is just another desperate attempt by these secret-money attack groups to distort Claire's record," McCaskill campaign spokeswoman Caitlin Legacki said Wednesday.
McCaskill will face the winner of an Aug. 7 Republican primary that features former state treasurer Sarah Steelman, U.S. Rep. Todd Akin and businessman John Brunner. The three Republican candidates all have highlighted McCaskill's ties to Obama. But Brunner also recently began airing TV ads criticizing Steelman and Akin for "manufacturing debt" through votes they took as state and federal lawmakers.