When Mike Pertschuk took over the chairmanship of the Federal Trade Commission 20 months ago, he promised to make it the "best public interest law firm in the country," and so far he's done his best to keep that promise.

In his short tenure, the former consumer advocate and chief counsel of the Senate Commerce Committee has turned the FTC into a crusader against consumer fraud and suspected corporate sins.

On the way he's reaised the blood pressure of funeral directors, vocational school administrators, doctors, insurance executives -- and even a federal judge.

Like other consumer advocates who joined the Carter administration, Pertschuk has used his appointment as a license to bring consumer interests into law enforcement, calling the FTC an advocate of the marketplace.

"We are essentially a very conservative agency," he said recently. "Most of what we're trying to do is free up the marketplace from restraints, both imposed by government and imposed by business. We are not in the business of subvstituting our choice for the consumer's choice."

Pertschuk's FTC has been attacked by the Business Roundtable, the National Association of Manufactureres and the Chamber of Commerce, which have tried to blunt Pertschuk's aggressiveness through legislation and lobbying pressure on the administration.

"He is heading the wrong was at the wrong time," said Jeffrey Joseph, a chamber lobbyist. "People want less federal involvement in their lives, not more."

The industry lobbying has has its effect. In passing the FTC's budget, Congress added an unusual proviso limiting certain FTC investigations. And some congressmen have hinted that in the coming session they will try to give Congress veto power over any new regulations the agency plans to propose -- an effort that has failed in other areas.

"The best way to gear up to fight these things," Pertschuk said, "is to make damn sure we don't make ourselves vulnerable."

Sounding more and more like a soldier who has just been thrown into the trenches, Pertschuk relishes the fight, since he is familiar with the turf on Capitol Hill. One thing he has done is put the FTC's house in order.

"For the first time our bureaus trust each other, and count on each other and the general counsel for information," he said. "In the past they were constantly fighting each other, and never trusted the information they would get from each other.

"We have also strengthened our regional offices and put in some strong regional directors. Our bureaus now take on cases of national impact on their own."

To see what goes on in a region, Pertschuk spent a week answering the telephones at his Chicago office.

Pertschuk has strengthened policy planning and evaulation, forcing the agency to allocate its resources in the most effective way. Monthly task force meetings review the progress of investigations and actions.

Pertschuk is also proud of having assumed an independent advecate role on certain issues. He openly opposed the administration on a bill to give U.S. shippers a guaranteed percentage of imported oil and on steel industry trigger prices.

A detailed list of substantive accomplishments of the FTC in the Pertschuk era reveals an active agency that is getting even ore aggressive.

In the area of health care, for example, the FTC is probing physician dominance of Blue Shield, competition in the Health Maintenance Organization field, the need for generic drugs, doctor's advertising, the hearing aid industry, and eyeglass advertising.

In moving against corporate power through the antitrust laws. Pertschuk's raiders are trying to set new ground rules for going after potential monopolies. In a case against Du Pont, the FTC charges that the company has built enought production capacity to dominate the market in titanium dioxide (a whitener for paint, paper and other products) for 10 years to come.

Other antitrust actions are planned against conglomerates and the energy industry.

The FTC is also actively intervening with other agencies to promote deregulation of trucking, railroads, airlines, trade policies, energy and comparative advertising.

And the FTC recently won a substantial victory against hundreds of corporations fighting the agency's attempt to secure "line-of-business" information from every major company. The information will be used for industry profiles and other economic studies designed to support agency actions, particularly antitrust.

And in two weeks, the FTC will hold public hearings in San Francisco on children's advertising, one of Pertschuk's most controversial initiatives.

It was an FTC staff report last February calling television advertising aimed at children "inherently" unfair and deceptive that inspired the strongest industry reaction to Pertschuk. He became so closely identified with the children's advertising issue that a federal district judge in Washington ruled that Pertschuk should disqualify himself from further participation.

The FTC has appealed the ruling, but the case has taught Pertschuk he must temper and manage his energy more effectively.

"Mike is very personably guy who has strong ideas about what he wants accomplished," said the Chamber of Commerce's Joseph. "Some of those... are things he couldn't accomplish when he was on the Hill, and you know what they say: if you can't legislate, regulate.

"He is one of many social activists and, like the rest, he wants to expand the powers he has in the government. He wants to extend his boundaries to the ends of the world and pick up everything in between."

As for the future, Pertschuk says the FTC did "all right" in its budget request, compared with other agencies that were cut back, and thus faces no major curtailments.

But there are short-term problems ahead. Pertschuk must make a case for the existence of his commission in the face of "sunset" regulations that force analysis of an agency before it is reauthorized.

"Every industry whose greed was restrained by the FTC has the incentive to try and trim us back," he says.