Spain's Socialist trade union roundly defeated Communist candidates in a crucial vote today for worker representatives at the nation's main automobile plant.

The upset victory by the moderate Socialist General Workers Union on the eve of May Day, Europe's equivalent of the American Labor Day, came in the first major vote in nationwide trade union elections over the coming weeks and indicated that Communist control over Spanish labor is waning. That influence had been built up clandestinely during the last years of Francisco Franco's dictatorship.

The Socialists gained 78 workers council posts representing the 32,000 labor force at the Spanish subsidiary of the Italian Fiat auto conglomerate, against 46 elected representatives of the Communist-led Workers Commissions. The result turned the tables on the Communist-backed candidates, who had previously held a 57-54 edge over the Socialists.

The vote at the eight plants, which are chiefly based in the industrial belt of northeastern Barcelona Province, was seen as significant by political circles since the auto workers traditionally have been at the forefront of trade union militancy and a bastion of Communist influence.

Greeted jubilantly by Socialist leaders, the result was roundly criticized by Workers' Commissions sources who said that there had been government interference and alleged that the administration had lobbied in favor of the Socialist Union. Socialist sources said the result indicated that the leadership of the Workers' Commissions over the Spanish labor force had marked a downward turn.

The battle for control of the working force between the two union organizations has come during the post-Franco dismantling of the dictatorships state-controlled vertical industrial syndicates.

The Socialist union has campaigned in favor of keeping within an anti-inflationary wage guideline negotiated with the Spanish employers' federation. That pact was denounced by the Workers' Commissions and the Communists stepped up strike action in recent months to back wage increases in excess of the agreed 16 percent limit.