GM and Chrysler's backlash against the Romney campaign is almost unheard of within the ranks of the Fortune 100. Is this a sign of corporate political activity to come?

By Anne VanderMey, reporter

FORTUNE -- In an election season that's given voters many firsts, we can add public sniping and fact-checking by blue-chip companies to the list.

Last week's dustup between the Romney camp and Detroit automakers started with what seemed like a pretty typical dig in campaign ad-inundated Ohio. Romney's ad: "Barack Obama says he saved the auto industry. But for who? Ohio or China?"

Claims that car companies are shipping American jobs overseas have been largely debunked by outside groups. What wasn't typical was the debunking directly from GM (GM) and Chrysler, putting a new twist on an already-acrimonious election cycle now in its final hours.

"Politics at its cynical worst," was the choice phrase GM spokesman Greg Martin used to describe the ad to The Detroit News. Sergio Marchionne, CEO of Chrysler's parent company Fiat, wrote in a memo to employees, "Jeep assembly lines will remain in operation in the United States and will constitute the backbone of the brand." He concluded: "It is inaccurate to suggest anything different."

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That kind of backlash from a publicly traded company is almost unheard of in contemporary politics. "I'm a historian and I get paid to think about this kind of thing, and I can't think of another analog," says Harvard Business School professor Nancy Koehn, who studies corporate leadership. "This isn't like some celebrity CEO saying, 'Hey, I'm going on a roadshow because I want to save animals.'"

These sorts of direct political statements are still rare in the genteel world of Fortune 100 companies. Most firms still work in a political system characterized by backroom dealing and soft-money corporate political donations routed through front groups. But as the lines between business and politics continue to blur, Americans can expect to see more public pronouncements. Indeed, it could very well be a "canary in the mine shaft" moment, Koehn says. More