President Raul Alfonsin's Radical Civil Union has won a solid victory in mid-term congressional elections, widening its slim majority in the lower house and gaining a solid endorsement of its economic austerity policy.

A nearly complete count of yesterday's votes showed the Radicals with about 43 percent, against about 34 percent for the combined Peronist factions.

However, Alfonsin's victory was by far less than the overwhelming margin some of his aides had wanted and expected.

The figures indicate a slippage in electoral support for both major political parties. But the Radicals have ended up widening what had been a thin absolute majority in the 254-member lower house.

Officials interpreting the election figures said the Radicals would pick up three or four additional seats. In contrast, the Peronists lost 10 house seats. The leftist Intransigent Party gained several additional seats, as did small rightist parties.

Despite their defeat, Peronist leaders found encouragement today in having prevented an even greater erosion in party support.

In Buenos Aires, the fight was kept within the party, with moderate Carlos Grosso, an educator, having wrested the nomination months ago from Lorenzo Miguel, a veteran union strongman. But in the province of Buenos Aires, the Peronists were forced to run two opposing lists.

Yesterday, the reformers triumphed. Analysts said their victory would help the Peronists to restructure the party in time to pose a serious challenge to the Radicals in the 1989 presidential elections.