Paris — After nine months of winning almost every political battle in sight, President Francois Mitterrand's Socialist government has been stung by two weekend setbacks.
On Jan. 16, France's Constitutional Council, the nation's highest judicial body, ruled that several provisions in the government's nationalization law are unconstitutional.
Then Jan. 17, the conservative opposition swamped the Socialists in four by-elections, the first such test since general elections last summer. Three Socialists lost their assembly seats.
''It's the finish of their state of grace,'' declared Lydie Gerbaud of the main neo-Gaullist opposition party, the Rassemblement pour la Republique (RPR).
Yet even Mrs. Gerbaud recognized that the opposition's victories were, above all, symbolic, at best jabs that could slow the momentum of the new administration.
The Constitutional Council did not overturn the nationalization of 11 major industries, but just the law's articles dealing primarily with the reimbursement of shareholders.
The ruling sent the government into strategy meetings that lasted through Jan. 17 and are expected to result in revisions in the law to be submitted to the entire Cabinet Jan. 20. ''The enaction of the law will be delayed,'' Prime Minister Pierre Mauroy confirmed Jan. 17, but he pledged that it would ''proceed with the government-announced nationalizations.''
The Socialist Party played down the significance of the delay. ''Since the judges on the council were appointed by past right-wing governments, their decision is not surprising,'' said Axel Queval, a party spokesman. ''The decision doesn't change much,'' he added, saying it will probably only result in an increase in the level of reimbursements.
Queval also dismissed the electoral losses as ''insignificant'' since they still leave the Socialists with a comfortable majority in the 491-seat national asembly.
Left-wing voters didn't turn out because they realized the elections were ''meaningless'' since the Socialists were so entrenched in power, he explained. As proof that the Socialists retain the confidence of the majority of Frenchmen, he pointed to the magazine Paris Match poll last week, which gave President Mitterrand a 59 percent approval rating.
Still, the center-right opposition won large majorities, around 55 percent of the votes in the four contested districts whose parliamentary elections last summer were invalidated because of irregularities.
All four of the districts have traditionally voted conservative.
Alain Peyrefitte, justice minister in the previous administration, swept his by-election and received congratulations from his former boss, Valery Giscard d'Estaing.
But the biggest winner seems to have been Paris Mayor Jacques Chirac, the leader of the RPR, who has been trying to consolidate control of the disorganized opposition. He called the victories ''very satisfying,'' as three of the four victorious candidates came from his party ranks.
''This will show the government that there is a strong and solid opposition, '' Mrs. Gerbaud said of the RPR said. ''While it will not force them to change their policies, it will slow down their revolution and increase their reflection.''