The Spanish Socialist Workers Party, the largest opposition group in the country, will drop its Marxist definition at a forthcoming extraordinary congress, paving the way for broader voter appeal, more moderate policies and the return of former leader Felipe Gonzalez.

The development, considered crucial to the implementation of parliamentary democracy in Spain, came as party organizers examined the debate motions sent in by the delegations that will attend the congress.While certain motions propose that Marxism be recognized by the party as a "valid, scientific instrument in the interpretation of reality," none specifically define the party as a Marxist organization.

It was the Marxist label that caused a previous congress, held in May, to end in confusion when Gonzalez, then secretary general, refused to stand for reelection unless the party changed its ideological premises. In a calculated gamble, Gonzalez forced the party to name a caretaker executive charged with holding an extraordinary congress next month to debate the issue.

The decision to drop the Marxist label -- which was adopted formally in a 1972 congress -- was seen as an effort by the party to moderate its policies and broaden its electoral contest. According to the constitution, elections must be held within the next five years.

The decision also was seen as a personal triumph for Gonzalez.

After a three-month "period of reflection," the often radical rank-and-file party members apparently have come around to the former leader's point of view. Gonzalez, at 37 a charismatic personality who is known nationwide simply as Felipe, had argued that Marxism was not only outdated and meaningless as a party definition but that also it scared off voters.

The internal debate carried out during the summer in party publications and at meetings, often spilling out into the national press, was bitter as extremists accused Gonzalez and his supporters of selling out to social democracy and West German influences.

Gonzalez, a labor lawyer, was first elected to the party's top executive post at a clandestine congress held in a suburb of Paris in 1974, a year before Gen. Francisco Franco's death and more than two years before the party was legalized in Spain.

Gonzalez and his close associates subsequently developed the party into a major party in the country, second only to the ruling Union of the Democratic Center headed by Premier Adolfo Suarez.

The Socialist Workers Party captured 118 congressional seats compared to the Union's 165, in the first democratic elections held in June 1977. But in the general election held last March, the Socialists failed to make inroads. Suarez was returned to power with 167 seats, compared to the Socialist Workers' 121, with more than half a million votes separating the two parties.

According to the Socialist Workers' post-election analysis, the party failed to carry the middle-class vote despite its carefully pitched campaign platform that sought to reassure voters on the issues of law and order and economic recession. The Union of the Democratic Center, meanwhile, fought an aggressive campaign, roundly accusing the Socialists of being dogmatic and trying to exploit the Socialists' Marxist definition,

When the Socialist Workers' congress opens in Madrid on Sept. 28, the motions to be considered are expected to adhere to prior basic tenets such as the nationalization of the means of production. Many motions still refer to the party as being class-based. The specific Marxist issue, however, was an important psychological barrier, and it is expected that subsequent congresses will lead the Spanish Socialists through an ideological evolution similar to that of the West German Social Democrats who, in their socalled Godesberg Program of 1959, accepted a democratic welfare state and a market economy.

Moderates in the Socialist Worker Party clearly feel that any defection by leftists to the Communist Party -- at present a poor third in the party stakes with 23 congress seats -- would be more than offset by attracting progressives from the UCD.

Central to the whole debate is that Spanish politics have become stabilized to the point where it has become all-important to gain the middle ground in party politics.