In a burst of political optimism, normally dispirited Guatemalan politicians have begun to campaign for next year's expected presidential elections even though the new constitution that will set the date and ground rules remains months from completion.

The premature presidential campaign reflects a new mood of hope that after decades of military-dictated government, President Oscar Humberto Mejia Victores means to keep his promise to turn over power to a freely elected civilian government next year.

Gen. Mejia Victores, who seized power from a previous military ruler on Aug. 8, 1983, held elections July 1 for an interim Constituent Assembly to write a constitution. Critics say that gave the military an additional year in which to establish policies before turning over the government to civilians.

Last week, Mejia Victores said that even though the constitution appears to be taking more time than had been expected, he remains confident that the assembly will finish its work by spring and that elections can be held by summer, possibly in July.

"I believe Gen. Mejia Victores is sincere when he promises elections," said Vinicio Cerezo, the head of the Christian Democratic Party and one of the leading presidential candidates. "I think the process is opening. There is more space politically, and the Army has come around to the view that it should get out of running the government."

Cerezo said he bases his optimism on the unchallenged honesty of the elections July 1 for the 88-man assembly. Other politicians and analysts interviewed in recent weeks expressed similar views. In a country long conditioned to military-engineered electoral fraud, the fact that all parties agreed on the rules and the outcome was seen as a portent.

Cerezo, 44, knows the hazards of politics in this country. In the 1970s and early 1980s, hundreds of his party workers, candidates and local office holders were killed by right-wing death squads, which in many cases were widely recognized to have been tied to state security services. Cerezo, who recently denounced the "disappearance" of four of his party workers, has been a target of assassins.

"I'm not saying we will have democracy when we have a constitution and a presidential election," Cerezo said in an interview, "but it will be a beginning."

The hope of politicians such as Cerezo hinges on their belief that the Army has recognized that it is faced with an economic crisis it cannot resolve: the country's reputation for human rights violations has so isolated Guatemala internationally that its chances of getting needed aid are slim.

Army commanders, according to diplomats, also feel they can step out of the direct operation of government more easily now that ruthless counterinsurgency methods have reduced the challenge from left-wing guerrillas in the Indian highlands.

According to economists here, the agricultural-based economy is a shambles. With international prices for its main exports of coffee, sugar, cotton and cattle depressed, the nation is so strapped that it has a $62 million deficit of foreign exchange, managing to pay for its subsistence imports only on the basis of loans. Servicing of the $2.3 billion foreign debt takes 37 percent of export earnings.

Diplomatic sources said the Army, too, had begun to feel the crunch of its decision in 1977 to reject U.S. military aid because of human rights conditions that President Carter sought to impose. As a foreign military official here put it, "After five years of guerrilla war on their own, they are down to rubber bands and baling wire."

"I think one of the reasons there is a change in thinking is the fact that the United States and Europe, who can provide the help Guatemala needs, are favoring democracies over dictatorships," said newspaper publisher Jorge Carpio Nicolle, who is a presidential candidate for the center-right Union of the National Center that he founded a little more than a year ago. "The new variable is that the population of Guatemala can no longer do without development, and we cannot get that under signs of repression."

Numerous left-of-center politicians say, however, that the Army knows it can step out of the presidential palace and remain the arbiter of politics, as it has been for the most part since the CIA supported a coup against leftist president Jacobo Arbenz in 1954. "I believe the Army will step aside," said one Social Democrat who asked that his name not be used, "but it will always remain a major force in politics. The Army knows it can step back in anytime it wants to, no matter what constitution we eventually adopt."

As worrying to some veteran politicians here is the possibility that elements within the Army accustomed to their perquisites might try to prevent the election before it occurs.

According to several Guatemalans with military contacts, the Army is divided on what the course should be. These sources said that while Mejia Victores and the officers who support him seem to be in the majority at the moment, at least three other factions exist. These sources allege that in the past year there have been several attempts to organize a coup against Mejia Victores that were averted only at the last minute.

"As the election nears," said one of the sources, a professional who has been active in politics here for the past decade, "the greater will be the temptation to derail it by those that oppose it."

Cerezo and Carpio, the presidential front-runners according to private polls they commissioned, say their hope is that the momentum for the election is already so great that it would be hard to abort. After that, they say in private, it will be up to whoever wins to govern in such a way as to discourage another Army intervention.

The Christian Democrats won 17.2 percent of the vote last July against the Union of the Center's 14.5 percent and 13.2 percent for the right-wing National Liberation Movement coalition headed by Mario Sandoval Alarcon.

Cerezo's center-left politics have always been a red flag to conservatives and the Army. Carpio is viewed as Cerezo's most important challenger, especially after he announced an electoral alliance with two other right-of-center parties this month.

As it will be at least six months before elections can be held, prognoses may be premature. Sandoval Alarcon, perennial candidate of the extreme right, is viewed as the third most important contender. His chances could rise if politics become more polarized in the coming months as the Constitutent Assembly debates the final constitution draft.

"The candidates are off and running," said a European diplomat here, "but there are still a lot of unknown factors, such as the constitution, the attitude of the Army as a whole, continuing political violence, that will dictate its ultimate outcome."