Given conventional wisdom, Robert Mosbacher should still be wandering the halls of the vast Commerce Department bureaucracy in a daze, defeated at every turnby unfamiliar government red tape.

How could someone new to the Washington power game, overshadowed by his dazzling socialite wife, possibly become a mover and shaker, the skeptics asked.Perhaps no other member of the Bush Cabinet took office with lower expectations than the president's choice for commerce secretary. But in just five months in office, George Bush's longtime friend has run up a string of successes. To wit:

-He prevailed over determined opposition at the Defense Department in getting a deal to develop the advanced FSX fighter with Japan renegotiated on terms more favorable to the United States.

-He persuaded the president to accept his position to put Japan, Brazil and India on a trade hit list, with the threat of stiff sanctions unless the targeted countries agree to drop trade barriers.

-Using blunt talk about the need for "a seat at the table," Mosbacher won concessions from the 12-nation European Community and a promise to listen to U.S. concerns as Western Europe moves toward a unified market in 1992.

While impressed by his policy clout, detractors contend that Mosbacher is exhibiting dangerous "protectionist" tendencies, trying to cure the country's massive trade deficits by erecting barriers that will hurt American consumers more than they will help American producers.

Mosbacher dismisses those complaints, contending that he learned economics well during his school days and fully understands the consumer benefits derived from an open global trading system. But he says economic theory has to be tempered with "the reality of the world."

"The goal is always free trade, but you can't have unilateral free trade. It doesn't work," Mosbacher said in a recent interview in his office, where he displays an embroidered pillow with the legend, "Free Trade Ave. Is a Two-Way Street."

Mosbacher's early successes have made the Commerce Department, long considered a backwater agency with little impact on big decisions, a player in Washington.

Mosbacher, 62, adopts an aw-shucks pose, saying that in his 40 years in the oil business he learned that "luck beats brains every time."

While he never held any government job before joining Bush's Cabinet, Mosbacher maintains that reaching a consensus in government isn't all that different from the hard bargaining that goes on in business.

"You have to sell your viewpoint when you are trying to get somebody to buy your deal or when you are buying someone else's deal," he said. "I don't know any business in the world that is more based on trading than the oil business."

But another reason for Mosbacher's rising star in government is undoubtedly his tie to Bush - a friendship that dates back 30 years to the time both were starting out in the oil business in Houston.

Mosbacher and Bush, both the offspring of wealthy Eastern families, chose to seek their own fortunes in Texas. Mosbacher was considerably more successful than Bush, amassing a net worth estimated at more than $150 million, making him the wealthiest member of the Bush Cabinet.

Mosbacher raised a cool $25 million in large contributions as Bush's chief campaign fundraiser last year.

On a table in Mosbacher's ornate office is a photo of a triumphant Bush, arms upraised, taken at the 1988 Republican Convention. Scrawled across the picture is the message, "Bob, What a night! I'd never have made it here without you. Gratefully, George."

Mosbacher, however, objects to any suggestion that he is playing on his friendship to score bureaucratic victories.

"Being an old friend of the president's hasn't hurt, but I am not sure, even if I wanted to, that it would affect his decision-making to one degree," he said.

Whatever the reason, Mosbacher's early gains have been a tonic for the sprawling 40,000-employee department he heads. Composed of a hodgepodge of agencies from the Census Bureau to the National Weather Service, Commerce has rarely before been at the center of policy debates.

But the agency does have the responsibility of administering the country's trade laws, not a small task in an era of $100-billion-plus trade deficits.

Those deficits, considered by many to be the biggest economic failure of the Reagan years, have increased worries about America's standing in the world and its ability to compete with trade powerhouses such as Japan.

Touching on those concerns in his speeches, Mosbacher portrays the competitiveness issue in almost life and death terms:

"If this nation surrenders to complacency, if we lose control of industries and production of products vital to our defense and way of life, at stake is not just American jobs and prosperity, but America's freedom."

But within the administration, there is an intense debate over the proper role of government in assisting American businesses to compete internationally.

One view holds that anything that smacks of an industrial policy, where government singles out certain industries for assistance, is anathema to the Republican free-market views on economic issues.

But Mosbacher has been canpaigning for a much more activist approach.

He is careful not to define his proposal as an industrial policy, but rather an "industry-led business-government partnership."