The centrist party that has guided Spain through its transition from dictatorship to multiparty democracy has split into five fragments, assuring general elections later this year that could bring the Socialist Party to power while also benefiting the conservatives.

The decision last week of former premier Adolfo Suarez--the party's founder--to fight the forthcoming elections at the head of a new ticket was the final nail in the coffin of the once all-powerful Union of the Democratic Center.

In power since the first parliamentary election following the 1975 death of dictator Francisco Franco, the united centrists gained 169 seats in the 350-member Congress in the last general elections held in 1979. The Democratic Center's congressional strength is now down to around 120 certain supporters, which is about parity with the Socialist opposition.

Socialist Party leader Felipe Gonzalez, 40, who is the front-runner in opinion polls, has labeled the administration of current Premier Leopoldo Calvo-Sotelo "paralyzed" and called for the dissolution of parliament when it reconvenes next month.

The splits in the Union of the Democratic Center that preceded the one led by Suarez included formation last month of the Popular Liberal Party and the Popular Democratic Party. Both attracted centrist Congress members and the two seek a broad conservative electoral platform in alliance with a minority right-wing party, the Popular Alliance group, which is headed by a dynamic one-time Franco minister, Manuel Fraga Iribarne.

Suarez, in launching his new Social and Democratic Center, sharply criticized what he claimed was a rightward drift of his old party under Calvo-Sotelo's leadership and the realignment of the defecting sectors within a conservative coalition. He said such a polarization brought upon its proponents "the extremely grave responsibility of relighting the destructive conflagration that has accompanied Spain's civil discords for the past 200 years."

As the premier picked by King Juan Carlos to accelerate transition to democracy, Suarez, now 49, made consensus politics the cornerstone of his bold dismantling of the Francoist regime.

Suarez, in quitting the centrist union, also charged that since he resigned from the premiership in January 1981--and more specifically since the military coup attempt that took place the following month--Calvo-Sotelo had been unable to resist encroachments from a reactionary establishment.

Completing the fluid and confusing picture of centrist politics is another breakaway group, the Democratic Action Party, which is led by a reform-minded former bureaucrat who is credited with radically overhauling tax legislation and introducing a divorce law.

Efforts by Premier Calvo-Sotelo to hold the Democratic Center party together have included the recent appointment of respected Congress speaker Landelino Lavilla as party chairman, but the last-ditch salvage moves have been greeted with skepticism by political commentators.

Resurrected fears of right-left confrontation in Spanish politics were borne out in recent local votes to elect regional parliaments. Fraga Iribarne's rightist Popular Alliance Party won a plurality in elections held in the rural northwestern area of Galicia, which had traditionally returned a centrist majority.

The Democratic Center lost half its voters and was beaten into second place. Last May, the Socialist Party won a startling outright victory in a regional vote held in the southern provinces of Andalusia and the centrists were pushed into third place by the Popular Alliance.

In impoverished Andalusia, the centrist support was split down the middle--voters aligning with Socialists and rightists. The Socialists fought on a platform of political change and the Popular Alliance stressed law and order and the defense of church, family and property.

Last week, Calvo-Sotelo said that when parliament met in September it would be in a "pre-campaign" atmosphere but he refused to be tied to a dissolution date. Politicians expect elections in late November.