DETROIT -- As an undergraduate at Villanova, Jim Musselman wanted to study business. His father ran a small company -- Musselman Advertising -- in Allentown, Pa. At Syracuse University law school, where he was president of his class and a graduate in 1982, Musselman stayed with it and took courses in corporate law.

Today, five years out of Syracuse, the Washington-based public interest attorney understands American business -- its ethics, power and attitudes -- as do few others. His education came by watching and confronting General Motors, the world's largest industrial corporation and a behemoth in dozens of American towns.

In a recent six-month period, Musselman traveled more than 20,000 miles visiting 18 Michigan communities where GM has plants. All are scenes of legal wars in which the company seeks to reduce its property taxes. All have citizens' groups calling the reductions either avaricious or devastating. All have been rallied and counseled by Musselman, who earns less than $20,000 a year as an attorney for Ralph Nader's Center for the Study of Responsive Law.

If GM succeeds in its appeal for tax reductions -- the cases are now before the Michigan Tax Tribunal -- municipal governments stand to lose nearly $50 million annually in revenue. The schools and the fire and police departments of Pontiac would be out $12 million, Flint $8 million and Warren $7 million. The reductions for GM -- a multinational conglomerate, now the nation's fifth largest military contractor, with profits of $8.2 billion for two recent years -- would range from 30 to 85 percent.

Musselman put the numbers together. He estimates the tax reductions would equal about two days of profits for GM and a year's worth of services for the towns. A company spokesman says, ''We want to pay our legal fair share of property taxes.'' Roger Smith, the GM chairman, argues, ''We're not trying to pay less taxes than we should. We're trying not to pay more taxes than we should.''

While the tax tribunal mulls over that one, Musselman continues to visit the ambushed towns. He speaks -- with respect and admiration -- about ''the fighters I've met.'' These are teachers, school administrators, union officials, town supervisors and others who had long resented GM's power over local economies but who now are uniting and battling back.

One of these is William Ayre, the township supervisor of Genesee in southeastern Michigan. Ayre is grateful to Musselman: ''He drew attention to the problem and he had his facts straight. I think GM is dead wrong. They show a lack of social responsibility.'' Ayre, a Democrat in office for nine years, estimates that his community of 25,000 citizens will lose $885,000 a year if GM wins. Genesee schools, which receive 80 percent of the taxes, would be hurt the most. So would public safety. ''We'll lose three policemen,'' Ayre said. ''That's a lot when your force is 10 full-time policemen.''

The supervisor spoke of life in the shadow of the giant: ''The laws in this state are written so that a huge corporation has tremendous advantage over a small municipality. Our budget is $2.5 million a year. For us to fight GM could cost us $1.5 million. We are trying to get some laws passed in the state legislature to help us, but GM opposes them.''

Musselman has been astonished at what he calls the corporate arrogance of General Motors: ''It's all legal, but they are almost to the point of civic extortion in how they are strong-arming the towns by threatening to close or relocating the plants if the reductions aren't agreed to. GM and Roger Smith both look on themselves as friends of education, yet here they are depleting the state's educational resources.''

Back in Flint, where GM was founded in 1908, plant closings and layoffs are twinned with record rates of suicide, spouse abuse and crime. Some 30,000 job-seeking people have fled the town in the past five years. GM now wants tax reductions that will mean an $8-million-a-year loss for Flint. This was on top of tax abatements from 1976 to 1986 in which the company won a 50 percent reduction of its taxes on $1.3 billion of property.

Two years ago, the Flint Journal could stomach no more. It asked editorially, ''How can GM bully its friends this way?'' Jim Musselman has been supplying an answer: GM can do it if the citizen-victims allow it to. Lately, he is finding the allowers are fewer: corporate power is meeting its match in citizen power.