Battle ready

March 21, 2013, 10:58 AM UTC

Missouri Sen. Claire McCaskill was walking through the Capitol when she stopped to have a word with the man who had recently tried to get her tossed from the building. Last summer U.S. Chamber of Commerce president Tom Donohue, arguably the nation’s most powerful business lobbyist, had signed off on $3 million worth of ads against McCaskill. Donohue’s group dropped its campaign when her Republican challenger, Todd Akin, made his infamous rape comments. McCaskill viewed Donohue’s incursion as more than an attack — it was a troubling rightward lurch by the Chamber of Commerce. Now, having secured reelection as a red-state Democrat, she spoke up: “I said to him, ‘You know, I’d love to have some time with your board to talk about ROI and the importance of moderation around here,'” she recalls, referencing the roughly $31 million in federally reported funds that the Chamber spent in the 2012 election, only 7% of which went to successful candidates. McCaskill isn’t holding her breath for that meeting. Under Donohue’s stewardship, she says, the 101-year-old group has become a partisan outfit, and is in fact undermining the very business interests it is meant to protect by preserving government-crippling polarization.

That the Chamber prefers GOP control is beyond dispute. It spent less than 1% of its 2012 war chest backing Democrats. But the group says its politics follow policy: It gives lawmakers notice when it is taking a stand on an upcoming congressional vote, then factors how they broke into a year-end score. Anyone who ends up below 70 on a 100-point scale and draws a credible challenger can expect to face a barrage of Chamber-funded ads. McCaskill has a lifetime score of 40. “There’s an opportunity for her to increase her score if she wants to support the business community,” Chamber spokesman Tom Collamore says.

McCaskill rejects Collamore’s premise. Business interests need more moderates like her, she argues, to avoid repeats of the reckoning the Chamber faced in 2011, when the Tea Party Republicans it helped elect almost precipitated a debt default. An ideological ranking of Congress by the National Journal, a nonpartisan magazine, placed McCaskill dead center. The Chamber’s scoring system, she says, reflects the group’s partisan bias.

A dogged former prosecutor, McCaskill says she is already making the case to executives that Donohue’s Chamber is a prime culprit regarding the lack of progress on a deficit deal. If she were on the group’s board, she says, she’d want him fired. Collamore’s response: “We never make our political and policy positions personal, and we’re disappointed when others do so.” But maybe the group is feeling chastened by its campaign flop. In recent weeks the Chamber has been working with the AFL-CIO on a framework for immigration reform.

This story is from the April 08, 2013 issue of Fortune.