THE 10th anniversary in office has not been kind to European leaders of late. Britain's embattled Margaret Thatcher was finally forsaken by her own party a year ago after 11 years as prime minister, and this year French President Francois Mitterrand marked a decade in power to cries of "Ten years is enough!"Yet here in Spain, Prime Minister Felipe Gonzalez Marquez shows every sign of hitting his 10th anniversary in office next year with a promising future. His temples are a little grayer, certainly, from a decade of Spain's sustained "economic miracle" and opening out of isolation, and from close elections and squabbles within his own Socialist Workers Party over the country's course. With parliamentary elections required by October 1993, some Spanish observers say "Felipe," as he is often known, is likely to seek other challenges. Speculation is strong here and in Brussels that he will be watching European Community President Jacques Delors - and French domestic politics - to see if Mr. Delors might be vacating his chair in Brussels and returning to Paris sometime after 1992. But others say that, barring such a tantalizing opportunity, Gonzalez could easily seek a fourth term, arguing that Spain's evolution into a functioning democracy and competitive economy with an international role is not yet complete. In fact, some Spaniards say one of Spain's problems is that, after more than a decade of democratic multipartyism, no clear successors have emerged. "After Felipe," one Madrid resident mused recently, "I have trouble picturing anything other than ... more Felipe." Although the Socialists lost their absolute majority in parliament in 1989, Gonzalez and his party stand little chance of suffering anything worse than a required coalition with smaller center-left parties in the next elections. Part of the reason is that the country's right wing, headed by the conservative Partido Popular (PP), suffers from an image problem. "The Spanish electorate continues to be much more demanding of the right than of the left, because of its historical association with the Franco dictatorship," says Juan Luis Paniagua, dean of the political sciences and sociology faculty at the University of Madrid. The PP's young president, Jose Maria Aznar, is making some progress toward breaking down the image of the old right and projecting himself as a national leader. But his party still lacks national support. One reason is that the right remains much more fractured than the left, cut up among parties associated with specific regions and lacking a truly national base. "Until the right moves beyond the autonomy issue and its limitation to regional politics," adds Mr. Paniagua, "it will have a problem becoming a genuine national force." In a number of Spain's 17 autonomous regions, autonomy is often the focus of conservative and center-right parties. By comparison, the Socialist Party continues to benefit from its image as the political force that brought Spain out of isolation and into Europe. "The key to the Socialists' success is their continued association with and intelligent exploitation of a message of modernization," says Luis Moreno, a political sociologist with Madrid's Institute of Advanced Social Studies. "People are generally satisfied with the country's direction; they generally see progress and that translates into a feeling that the Socialists delivered on a promise." Somewhat ironically, it is from within the Socialist Party that Gonzalez is feeling the most domestic heat. As he has taken the party from Marxist, anti-American ideals to market economics and a pro-NATO stance, a rift has developed between the party's left and right wings. But Paniagua says the divisions are not as ominous for Gonzalez as they might appear. Even as former Deputy Prime Minister Alfonso Guerra "represented the unions, the populism to the left of Gonzalez; he created a balance." That same balancing act is going on now, he adds, "which probably puts the [Socialist] Party in the center, where the majority wants it, and where it can win." Despite Gonzalez's preparatory work, some analysts here say, the country's protected industries, inefficient agriculture, and poor, rural regions mean Spain is in for a shock with the arrival of the EC's single market after 1992. Gonzalez is currently trying to convince the community's wealthier members that they must do more for the poorer members if they want a cohesive, integrated Community with similar living standards. Spain is threatening to veto EC moves toward closer political integration, unless such an agreement includes more money for poorer countries. (Spanish per capita income is less than three-quarters of the EC average.) But most observers say if Felipe wants a fourth term in office, he will get it. "I have a hunch he'll go for another term; he's the Socialists' guarantee of a good result," says Mr. Moreno. "Without him, the outcome would be quite different."