Francois Mitterrand became president of France by turning upside down the Communist technique of the popular front. He formed a Union of the Left which, in the recent elections here, enabled his Socialists to emerge as a majority party at the expense of their Communist partners, who lost half their seats in the Assembly.

But can a technique that worked in opposition also be applied in office? That is the question that must be asked about the inclusion of four Communist ministers in the new French government.

The answer begins with the recognition that a wedge has been driven between the electorate and the apparatus, or machinery, of the French Communist Party. Communist voters, especially those raised during the prosperous years of the past two decades, found Mitterrand's call for social justice and human dignity far more appealing than the constant stress of the party hacks on class warfare. They turned against the party machinery when -- for purely technical reasons and because of Soviet national interest -- it knifed Mitterrand during the parliamentary elections of 1978.

In the first round of the presidential elections on May 14, millions of Communist voters deserted their party to back Mitterrand. The drop in the Communist vote (from 20 to 15 percent of the total) certified Mitterrand as perhaps the most effective anti-Communist on the world scene. In that role he attracted a flood of votes from the center that brought him victory in the final round of the presidential election.

The process repeated itself in the parliamentary elections of June 14 and 21. Socialist candidates routed Communists -- especially in Communist districts -- during the first round. In the second round, center votes gave the Socialists an absolute majority -- 269 of the 491 seats in the Assembly. The Communist total plunged from 86 to 44.

By bringing Communist ministers into the government, Mitterrand keeps up the pressure on the fault line between the party voters and the party machine. If Communist organizations, particularly the labor unions, go along with the government, Mitterrand has that much more chance to succeed. If they sabotage the government, they will stand accused in the eyes of their own followers of betraying the cause. So by the standards of the immediate past, Mitterrand's bet is "Heads I win, tails you lose."

But a big difference separates what worked in opposition from what is feasible in office. As president, Mitterrand has conferred upon a Communist Party, notorious for slavishness to Moscow and the practice of Leninist tactics internally, an acceptability, a legitimacy it could never have won for itself. The French example thus favors the entry of far less obnoxious communist parties into governments in Italy, Spain, Portugal and Greece.

In opposition, moreover, Mitterrand could easily outbid, or at least match, the Communists in all domestic issues. Thus he went much further than the Communists in supporting student radicalism back in 1968. He matched the Communists in pushing for higher wages, improved social conditions and nationalization of industry this year.

In office. however, positions count. When the Communists push for wage increases or more social services or nationalization, Mitterrand will have to count the impact on inflation, and the value of the franc, and business confidence, and French competitiveness on the world market. So the Communists, in theory at least, can outbid him.

Working against such tactics, however, are the authority and prestige of the president himself. Under the constitution cut to the measure of Gen. Charles de Gaulle, Mitterrand commands as president a broad range of extraordinary power over the ministers, the parliament, the civil service, the means of communication and both the public and private sectors of the economy.

As a leader backed by a majority of all French voters, and with a party that commands an absolute majority in the assembly, Mitterrand has more freedom to work his will -- more real power -- than any French leader since de Gaulle during a brief period in 1962. Maybe more than any French ruler since Napoleon. Given his past record, it strains credulity to believe he will not do his utmost to frustrate and thwart, to pulverize and atomize, to de-Leninize the French Communists.

So to me, the Mitterrand bet makes good sense. Though Washington has a right, even a duty, to assert reservations, the over whelming interest is to maintain rapport with the leader who had emerged as probably the central figure in Europe.