(FORTUNE Magazine) – Archer Daniels Midland has had several brushes with the law over the years, but none abrasive enough to halt its steady climb into the top ranks of American industry. Now, however, the agribusiness giant in Decatur, Illinois, is facing a federal probe that could shake the company to its roots and derail its brilliant, influential chairman, Dwayne O. Andreas.

Like a creeping vine, Andreas has insinuated himself into the confidences of Presidents Nixon, Bush, and Clinton; Russian leaders Gorbachev and Yeltsin; and powerful legislators such as Senate Majority Leader Robert Dole.

The vine has grown suddenly poisonous, and some people in Washington may soon be feeling itchy. Kevin Phillips, a noted political analyst and author, says Dole's opponents in the presidential primary, perhaps Pat Buchanan, could hurt the Senator by accusing him of being in the pocket of Andreas and big business. The Andreas family and the company have funneled hundreds of thousands of dollars to Dole's campaigns and favorite charities, and Dole in turn has been the architect and staunch defender of the 54-cent-per-gallon federal subsidy for ethanol, about 65% of which is produced by ADM.

The prospect of criminal indictments may force ADM's board members into action, particularly ones not named Andreas. The board includes the chairman's brother Lowell, son Michael, nephew Martin, and pal Robert Strauss, whose law firm, Akin Gump Strauss Hauer & Feld, has represented ADM. But the board also counts Shreve M. Archer Jr. and John H. Daniels, retired chairman, whose families helped found the firm. One possible scenario has Archer and Daniels leading a movement to replace Andreas with board member H.D. Hale, CEO of ADM Milling Co.

The drama in Decatur opened like a cop movie: In the evening of June 27, federal agents fanned out through the town (where Abe Lincoln once practiced law) and delivered subpoenas at ADM's headquarters and to key executives at their homes. The feds are snooping for evidence that ADM conspired with competitors to illegally set prices for some ag products, including high-fructose corn syrup, citric acid, and lysine, an additive to poultry feed.

Ordinarily, conspiracy is a crime devilishly hard to prove, but in this case the government has a dream witness: Mark Whitacre, 38, hotshot president of ADM's BioProducts division, who secretly gathered evidence for the FBI for three years. Whitacre's motive? "He's a straight arrow," says a friend. Ironically, Whitacre, his wife, and four children live in a house once owned by Andreas.

According to a source close to the company, the G-men have a smoking gun: videotapes of meetings in which senior executives discussed prices with the Japanese producers of lysine, Ajinomoto and BioKyowa. As late as 1989, the two Japanese firms were the kings of lysine (an amino acid derived from corn that promotes muscle growth), and the price was about $2.50 a pound. Then ADM opened a plant in Decatur. The price plummeted to 60 cents a pound in early 1993, and ADM captured 50% of the market. Since then prices have been gyrating between 60 cents and $1.50, according to Chemical Marketing Reporter.

Although ADM calls itself "supermarket to the world," it often behaves more like a banana republic, wallowing in intrigue and politics. Andreas emphatically denies that he or his company lobbies politicians, but they've donated millions to candidates of all persuasions.

Surely, ADM is no stranger to mischief. A $25,000 cash donation Andreas made to Nixon's presidential campaign in 1972 somehow turned up in the bank account of a Watergate burglar. In 1976, ADM pleaded no contest to charges it short-weighted grain exports. And in 1978 the company was convicted of conspiring to fix prices on products sold to the federal government's Food for Peace program. None of those scandals hampered Andreas's clout for long.

This one is far different. A period of confusion lies ahead for the supermarket to the world. And there will likely be casualties: The floor is littered with banana peels.

- Peter Nulty and Ronald Henkoff