A one-time enfant terrible of the Socialists' extreme left wing is overseeing multibillion-dollar government spending on research programs designed to turn France into a world scientific power and himself into a leading contender to be President Francois Mitterrand's successor.

As Jean-Pierre Chevenement, at 43 minister of state for research and technology, has moved from the doctrinaire past to the moderate present, he has shucked off the trappings that once made him the scourge of the conservative, down-to-earth middle classes.

Gone are the photograph of Karl Marx he used to keep in his office in his constituency of Belfort and the corduroys that were once his trademark.

There is just a bit of gray in his once raven locks and he wears a three-piece suit to receive visitors in his enormous, high-ceilinged ministerial office.

As the decor and clothing have changed, so have his political stands. Formerly an outspoken critic of the United States who said that the Soviet Union's military power was deployed only defensively, Chevenement recently maintained that "we are, and remain, loyal partners of the Atlantic Alliance."

"I am for national independence," he likes to explain in patriotic terms no Gaullist would fault, and he has sought to widen the Socialist net to include Gaullists and other disaffected nationalists.

For those who have followed Chevenement's career over the years, neither the successive transformations nor the abiding ambition come as any surprise.

About 20 years ago, for example, as a recent graduate of the National Administration School, that quintessential breeding ground of top civil servants and incipient politicians, Chevenement asked his elders for advice about which political party to join.

His interlocutors told the son of modest provincial elementary school teachers that the well-ensconced Gaullists looked like the best bet since the old, discredited Socialist Party was a mere shell.

"Precisely," Chevenement is reported to have said--and joined the moribund Socialists.

In 1966, with a handful of friends, he founded CERES, a study group that rapidly became the most radical faction of the Socialist Party.

Right-wing critics at the time said CERES' positions were difficult to distinguish from those of the Communist Party.

Chevenement and his friends turned on the old Socialist Party leadership and helped outsider Mitterrand win his takeover bid in 1971.

Chevenement favored Mitterrand's long-shot strategy of alliance with the then more powerful Communists.

But soon Chevenement was on the outs with Mitterrand and the majority of the revitalized Socialist Party, which he denounced as a "false Communist Party with real petty bourgeois."

But in complicated maneuvering among the rival Socialist clans, Chevenement rallied to Mitterrand and helped him beat back Michel Rocard's challenge to party leadership in 1979.

Such crucial help was rewarded when Mitterrand won the presidency last May and entrusted Chevenement with his present "superministry."

"You should be the advocate of the future," the president said. Chevenement took to research and technology with all the zeal of a convert determined to found a new rationalist religion on science and its practical applications.

As proof of his major status, Chevenement extracted a large budget commitment and the Hotel de Clermont, perhaps the most handsome of the former aristocratic Faubourg Saint Germain residences that serve as government ministries.

Chevenement took over all the scattered government research, both theoretical and applied. His budget last year jumped 30 percent. This year, and every year until 1985, he plans a 17.8 percent increase in real terms. That would bring research up from the 1.8 percent of the gross national product he inherited to 2.5 percent.

In the process, French research outlays would rise to the level of the United States and Japan.

From a high point in the Gaullist 1960s, funds earmarked for French research had fallen behind those in the United States, Japan, West Germany and Britain, all of which today stand at roughly 2.2 percent of their gross national products.

Chevenement has concentrated on so-called filieres, or product lines that cover vertical chains of production from raw materials to the finished product. Special emphasis has been placed on chemicals, electronics, the health field and its applications and new materials.

This month, Chevenement presented to the press a report that stressed that "if France should practice a global strategy on just one single industry she should choose the electronics filiere."

Although not yet official government policy, such talk raises fears among France's American, European and Japanese competitors that the Socialists are planning to freeze out foreigners, who now hold 30 percent of the French electronics market.

William Brock, the White House trade representative, while visiting in Paris recently, warned in purposely vague terms of just such protectionism in high technology. Although he named no names, he was widely thought to have France in mind.

Chevenement also has demonstrated his showman's touch in putting on a series of research-and-technology symposiums in the provinces, culminating in January in an attention-grabbing national colloquium presided over by Mitterrand.

Britain's Nature magazine took Chevenement and his plans seriously enough to devote 20 pages to his ambitions for French sciences. Although genuinely impressed by France's past accomplishments and future plans, the magazine compares the Super Phenix fast-breeder nuclear plant with the supersonic Concorde, both exceptional but costly technical successes.

Like his counterparts the world over, Chevenement is aware of the difficulties in finding practical industrial applications for fundamental research.

His solution lies partly in the Socialists' controversial nationalizations, which have brought almost all banking and much high-technology industry into the public sector.

Challenged to justify plans for massive government investment during the current depression, he recently said, "That is what makes the difference between Socialist reasoning, which attempts to anticipate, and liberal reasoning, which trusts spontaneous market forces." Socialist doctrine indeed justifies the nationalizations on the ground that French banking traditionally sought quick financial return and underinvested in industry.

But it is Chevenement's recent careful cultivation of the political middle ground among rival Socialist clans in particular and in French politics in general that has attracted as much attention as his ministerial stewardship.

The right-wing publication Le Figaro Magazine devoted a two-page color spread recently to Chevenement with a teasing headline, "Is This Ex-Leftist Mitterrand's Heir Apparent?"

In quoting from Chevenement's pronouncements past and present, the magazine noted his new-found faith in NATO and European unity .

Once an outspoken opponent of the European Community, he blithely told an interviewer, "Today, I am European because we are in power," apparently suggesting that only Socialists know how to build European unity.

More significantly in terms of practical politics, in recent months Chevenement has backed the implied criticism of Socialist stewardship voiced by Finance Minister Jacques Delors and Planning Minister Rocard, once denounced as belonging to the moderate or "American left."

Last fall, Chevenement tried to moderate radicals at a Socialist Party congress in Valence that called for "heads to roll" in the civil service because of their alleged foot-dragging allegiance to the ousted conservatives.

He also backed Delors' plea for a "pause" in the pace of reforms. After Socialist setbacks in local elections this spring, Chevenement endorsed Rocard's outspoken analysis of the defeat, which raised hackles in Prime Minister Pierre Mauroy's entourage.

"I believe the country needs authority," he said in a radio interview, adding that the government must "know how to say no."

Many Chevenement-watchers remain unconvinced about the sincerity of his apparent change of heart. They note that his closest advisers are mostly Socialist left-wingers or friendly to the Communists.

Quoting from various Chevenement utterances, left-wing philosopher Bernard-Henri Levy recently concluded that Chevenement was "a real man of the right masquerading in Socialist clothing."

But whatever the real Chevenement was, is, or will turn out to be, even his most determined critics credit him with major political talent. One such critic noted, "What a stroke of genius to have chosen a ministry where the results of his present policies--good or bad--won't become apparent for 10 more years."