Like thunderheads that pile up over midwestern prairies on summer afternoons, clouds have suddenly appeared on the horizon of Wisconsin Attorney General Bronson C. La Follette, heir to one of the state's proudest political legacies.

The State Ethics Board is investigating allegations that La Follette -- grandson of the founder of the American Progressive Movement, son of a U.S. senator and nephew of a governor -- intervened in questionable ways to quash moves by his own Justice Department against businesses to which he had personal or financial ties.

La Follette, 50, denies wrongdoing and says he is confident that the inquiry will "clear the air."

But the controversy has altered the political equation for La Follette, a popular Democratic vote-getter who is seeking a fourth consecutive four-year term. In 1982, Republicans did not contest his reelection. Now, two GOP candidates are vying for their party's nomination in a Sept. 9 primary, with La Follette's conduct in office certain to be a fall election issue.

The controversy involves his friendship with prominent state legislative lobbyist James Boullion, his private business dealings and his ties to a former assistant who sought approvals for a company doing business in the state. Questions also have arisen about La Follette's regard -- or lack of it -- for "appearance" as the state's top law enforcement figure. He oversees 90 lawyers, 65 investigators and prosecutes consumer fraud and other issues.

Most of the disclosures have surfaced in state newspapers. La Follette blames "a disgruntled employe" for leaks aimed at distorting his record, but the attorney general's behavior in the flap has raised questions as well.

La Follette initially opposed an Ethics board probe. As the stream of allegations continued, he demanded one. He complained, in an interview in his capitol office, that his attempts to set the record straight had been thwarted by an antagonistic press.

The board, which has hired a special counsel and a retired FBI agent to conduct the inquiry, expects to make a preliminary finding by mid-September. It could exonerate him or recommend civil or criminal litigation.

La Follette owns 4 percent of a financially troubled company headed by Boullion, Superlog Ventures Inc., which makes artificial fireplace logs. According to news accounts, Boullion arranged for a $35,000 loan to Superlog from a mausoleum developer who had retained lobbyist Boullion as an adviser. At the time, the developer was under investigation by La Follette's department for alleged criminal consumer fraud.

La Follette resigned from Superlog's board of directors when he learned of his department's investigation. The case is pending.

Meanwhile, another firm under state investigation for alleged illegal pyramid sales techniques is reported to have offered Superlog products as a promotion last year. La Follette says it was done without his knowledge.

Assistant Attorney General Stephen Nicks also alleges that La Follette dropped a lawsuit against another cemetery-space sales firm after Boullion intervened.

La Follette acknowledges that he quashed the case but denies that it was a favor to Boullion. He says he exercises prosecutorial discretion on a case's merits and has ruled against other Boullion clients. Boullion has said he exerts no influence over La Follette.

Allegations also have been made that La Follette dropped a Medicaid fraud case against a Wisconsin nursing-home operator after meeting with a former chief Justice Department aide who had served as his campaign director and later became lawyer for the operator. La Follette says he did what was right.

News media scrutiny of La Follette's conduct has extended to his 1985 federal tax returns, which he released last month. The returns show that he and his wife, Barbara, an assistant state health and social services director, earned $76,455 last year and claimed a taxable income of $29,055, taking big deductions from mortgage interest payments and business losses.

Deductions include a $10,882 Superlog loss. The returns also indicate that La Follette in 1983 and 1984 participated in a San Francisco tax shelter that the Internal Revenue Service last year filed suit against as an "abusive tax shelter."

La Follette, who describes himself as an "easy-going type" seems an unlikely figure for financial maneuverings. His grandfather, Robert M. (Fighting Bob) La Follette, rose to national prominence at the turn of the century by taking on banks and big business in Wisconsin.

He became a key Progressive Party figure and was a U.S. representative, governor, senator and Progressive presidential candidate in 1924, carrying Wisconsin in an election won by Calvin Coolidge. He died in office in 1925, and Bronson's father, Robert M. (Young Bob) La Follette Jr., succeeded him at age 30, the youngest man since Henry Clay to serve in the U.S. Senate.

Young Bob won two full Senate terms as a Progressive, while his brother, Philip, served as Wisconsin's Progressive governor for several terms in the 1930s. Young Bob's career ended in 1946, when he was defeated for the GOP Senate nomination by Joseph McCarthy.

One of two sons, Bronson grew up in Washington, was graduated from the Landon School and received undergraduate and law degrees from the University of Wisconsin.

His father committed suicide in 1953. La Follette's brother is a businessman in New York. But Bronson renewed the family tradition in 1964, winning the attorney general's post as a Democrat. He was reelected in 1966 but lost a race for governor in 1968 to GOP incumbent Warren Knowles.

He reappeared in 1974, won the job again, and has stayed in office ever since, echoing his family's Progressive roots by styling himself "lawyer for the little man." Recent polls show him with wide support.