On the eve of the second elections since the death of Generalissimo Francisco Franco in 1975, the political center seems to be the most crowded place in Spain.

Even the Spanish Communists have been trying to crowd their way into the center in an electoral campaign notable for its stress on glamor and gimmicry rather than issues.

It has been the kind of campaign for which Europeans traditionally mock Americans as proof that there are no real ideological differences in the United States.

One of the few meaningful slogans here was produced by an extreme leftist party unlikely to win any parliamentary seats: "Vote for the left that does not compromise with the right."

Even the traditional Spansih political violence outside theterrorism-wracked Basque country has been confined largely to the extreme left and the extreme right fighting each other. The bloody results only increase the general attractiveness of the center.

This is a perhaps inveitable byproduct of Premier Asolfo Suarez's effort to associate as many groups as possible in the consensus that has dominated the country during the two years that it took to write a generally acceptable text for a democratic constitution.

In the first election under that constitution, however, the issue, basically undebated, has been what kind of political system Spain will have in practical terms -- a two-party system, a multiparty system with revolving coalitions or continuation of the consensus politics that has led elsewhere in Euripe to extra-parliamentary extremism or the enhancement of the Communist party.

Premier Suarez, exasperated by his inability to chase his opponents from the center into which he invited them, complained in his final televised appeal last night about Socialist attempts to run away with the moderate image.

"We do not believe in the centrist moderation which (The Socialist Party) has tried to project," said Suarez. He accused the party of muting its advocacy of abortion and economic collectivesm and its opposition to church schools.

A photo montage showed Suarez with Cuba's Fidel Castro then a fade-in to Suarez with President Carter.

Socialist leader Felipe Gonzalez preceded Suarez on TV, saying his party is ready for governmental reponsibility, favors firmness against terrorism and seeks the support of the armed forces. "The Spanish people are not communist," he said in a dart aimed leftward. Suarez's refused a televised debate.

Parties have spent millions to say nothing much, littering the streets of Madrid with thousands of slick-paper leaflets dropped from planes with little more than the party names and symbols as a message.

The most widely dispayed posters have been those of the 45-year-old Suarez, showing his gleaming teeth in a smile, and the 36-year-old Gonzalez, showing that his boyish shock of hair is beginning to show traces of gray.

Aside from the question of how the central government can deal with the danger-laden demands for regional autonomy, the overriding issue is whether the electorate will produce a result that points Spain toward a vigorous two-party system or creates a situation in which the only effective way to govern is for the two main parties to do it together.

That course is already seen here as a likely outcome. Neither the Socialists nor Suarez' Center Democratic Union may get enough lower-house seats to govern by itself, yet the smaller parties -- except for the Communists -- are being so weakened that they may not be able to provide the margin of parliamentary control.

In the last election for the 350 seat lower house in 1977, the Center Democrats won 165, the Socialists 118, Communists 20, Conservatives 16 and the rest scattered.