FEDERAL laws passed during the Watergate era were supposed to put strict limits on campaign contributions by individuals, and to cut off campaign spending by corporations.So in 1989-90, why was Mrs. Mary C. Bingham, whose family once owned the Louisville Courier Journal, able to contribute $281,000, most of it to the Democratic National Committee? Why were actor Bill Cosby and his wife able to give $100,000 to Democratic Party organizations? Why were groups affiliated with the Atlantic Richfield Company able to pump $308,360 into the Republican and Democratic Parties? The answer: "soft money" loopholes that are helping business, labor unions, and wealthy individuals steer around official limits on contributions. A study released Monday by the Center for Responsive Politics found that during 1989-90, companies, unions, individuals, and other groups donated more than $43 million of unregulated funds to the major political parties. The study was not comprehensive. It covered only the two national parties and their affiliates in nine states. If the other 41 states were surveyed, the totals would be much higher. Joshua Goldstein, who wrote the study, says the growing levels of soft money (that is, money not restricted by federal law) represent a "significant loophole" in campaign regulations. In an interview, Mr. Goldstein said the rapid rise in soft-money donations since 1980 appears to be giving wealthy individuals and businesses a larger role in both presidential and congressional campaigns. Do these large contributions harm the political system? Goldstein offers no firm conclusions, but he says the evidence is not encouraging. Goldstein notes that the two major parties are making efforts to "go after the people who can give $100,000 each. And those people are obviously not average citizens.... I think the result is that the average voter is out of the loop, and people who make decisions about what the party stands for are not the voters, but those who give a lot of money."