As the three Republican candidates have battled it out, Ms. McCaskill has had to buckle down as well. Karl Rove’s Crossroads GPS, David and Charles Koch’s Americans For Prosperity, the U.S. Chamber of Commerce and the 60 Plus Association have dumped as much as $15 million into the state since July 2011 to keep her on her heels. In their advertisements, Ms. McCaskill’s face is sometimes bloated, sometimes goofy, sometimes exhausted. She is usually joined at the hip with President Obama. And she is always almost single-handedly to blame for Missouri’s economic travails, the nation’s skyrocketing debt, the Democrats’ health care law and a scandalous level of duplicity. “As one guy said to me in rural Missouri, ‘Don’t worry, they’re trying to tenderize you before they pick a candidate,’ ” Ms. McCaskill said Thursday. The sustained campaign could become a textbook for future efforts in a new era of anything-goes campaign financing, both Ms. McCaskill and her opponents say. Most of the spending is coming from tax-exempt 501(c)(4) organizations like Crossroads GPS, which may accept large corporate and individual donations without disclosing donors’ identities. And the outcome could show that third-party advertising from these organizations and from “super PACs” — like Now or Never, which works on behalf of Sarah Steelman, one of the Republican candidates — could tip the balance to a larger degree in a statewide or Congressional race than in the presidential contest. In other states with contested Republican primaries, like Arizona, Indiana and Wisconsin, outside money flowed in to take sides in the primaries themselves, leaving the Republican contenders wounded and the Democrat in better shape. In Missouri, it flowed in largely to take down the Democrat, providing vital air cover while the Republicans fought each other below the radar. The three Republican candidates may be lesser known and less dynamic than Ms. McCaskill, a mainstay of Missouri politics, but she is possibly the most endangered incumbent in the Senate. A Mason-Dixon poll published on July 28 in The St. Louis Post-Dispatch found her trailing all three of her potential opponents, John Brunner, a businessman; Ms. Steelman, a former state treasurer; and Representative Todd Akin. “When you have a late primary like in Missouri’s, the ability to keep a sustained message-fire on the incumbent is going to be important to whoever the nominee is going to be,” said Kenneth Goldstein, president of the Campaign Media Analysis Group, which tracks campaign television spending. Republicans are reluctant to give the outside groups too much credit. They say Ms. McCaskill’s uphill climb to a second term is due to her fealty to Mr. Obama’s legislative agenda in a state where he is unpopular. “Certainly the outside spending has reminded voters why they might not like her much,” said Todd Abrajano, an aide to Mr. Brunner. “But if there was no outside money, she’d still be in the predicament she is in today.” But when pushed, Republicans do not deny that the groups have helped. “They are keeping the pressure on McCaskill,” Mr. Akin said. “And that’s making any of us think, ‘We can do it, we can do it.’ ” Senator John Cornyn of Texas, the chairman of the National Republican Senatorial Committee, allowed, “I can’t deny it’s had some impact.” The Missouri Senate race has drawn outside money from more than a half-dozen groups: two Democratic outfits, the Majority PAC and Patriot Majority USA; and five Republican allies. The Campaign Media Analysis Group tallied $5.2 million in ads since June 1, compared with $4.8 million in Ohio, $2.9 million in Florida, $1.5 million in Montana and $1.1 million in North Dakota, where other contested Senate campaigns have drawn outside attention. The Majority PAC has spent $722,000 since June to help Ms. McCaskill. Crossroads GPS has spent $857,000 against her, and the U.S. Chamber of Commerce $190,000. Democratic strategists say those numbers understate the impact. Crossroads GPS’s first ads ran in early July 2011, and since then, nine separate ads have come in a rolling barrage. When Crossroads GPS eased up in June, the chamber peaked in July. By the Democrats’ count, anti-McCaskill spending has already reached $15.2 million, with an additional $18 million in advertising slots reserved for the fall. “People will look back at Missouri and look back at the money that’s been spent by outside forces and say this is an election that shows whether or not these anonymous masters of the universe can buy these elections,” Ms. McCaskill said. “If it works in Missouri, then I think we’re in for a rough ride in this country.” Her opponents dismiss that. Since Ms. McCaskill’s narrow Senate victory in 2006, her state has drifted right. Mr. Obama lost Missouri in 2008 by fewer than 4,000 votes out of nearly three million cast. But in a 2010 Senate race, Roy Blunt, then a Republican representative, crushed Robin Carnahan, a Democratic scion of Missouri political royalty, by nearly 14 percentage points. Some Missouri Republican strategists said the outside groups knew that Ms. McCaskill could not win and flooded the state to claim a scalp to take to their donors. Others said they saw her weakness and decided they could not forgive themselves if they let her off the mat. For her part, Ms. McCaskill has made the outside money the main opponent of the campaign. One McCaskill advertisement says: “They just keep coming back. Secret money attacking Claire McCaskill. These big oil and insurance companies don’t want you to know who they are.” As a stream of televisions showing her competitors’ ads moves across the screen, it continues: “Claire McCaskill will fight them. Always has, always will.” Republicans say her effort has proved to be a major strategic error. Ms. McCaskill’s focus on the money has only intensified awareness of the campaign’s national importance, made her look defensive and increased the resolve of the outside groups to stay in, they say. But Ms. McCaskill said that once her opponent was chosen, she could shift gears and try to move the race from a referendum on her to a choice between her and her opponent. “I’ve had to tread water,” she said, “while they have been pounding me with certainly more per capita than anybody else in the country.”
