With as little publicity as possible, the first contingent of U.S. Air Force personnel has arrived in this rural Sicilian town to man the cruise missile base under construction here. It is expected to be a major campaign issue for the leftist opposition in national and local elections next month.

According to a U.S. Embassy spokesman in Rome, more than 200 servicemen have arrived this month. Deployment of 112 mobile cruise missiles is to begin at the end of the year, barring a disarmament agreement between the United States and the Soviet Union.

By 1987, there are to be as many as 1,900 civilian and military personnel, not including dependents. The initial arrivals have trickled into the area in small groups.

Nevertheless, with local and national campaigns heating up for the June 26 election, the influx of U.S. servicemen has not gone unnoticed. Their arrival has provided a focus for renewed opposition to the base. Arguments cited against it include fears of social disruption, growing drug use and prostitution. Others make the claim, at least partially supported by Italian government authorities, that an upsurge of Mafia activity could result.

The Communist Party, the largest single political group in Comiso with 43 percent of the vote, is opposed to the base. Socialist Mayor Salvatore Catalano, who heads a city coalition with the nationally dominant Christian Democrats and the small Social Democratic Party, backs the base both on grounds that the missiles are necessary to the West's defense and that the U.S. military presence will bring significant economic advantages.

Comiso's population is less than 28,000. Scores of grizzled men gather in the main square each evening to gossip and discuss sports, politics and women. To an outsider, it appears a haven of tranquility and tradition. The area historically has been free of Mafia crime. Women keep out of sight, right- and left-wing party headquarters stand peacefully side by side, and a building boom suggests recent prosperity in truck farming and quarrying. However, the current Italian recession has left several hundred unemployed.

The Americans already assigned to the base are keeping a low profile. They go by bus to nearby beaches. At the smallish base, construction at the former Magliocco military airport is still concentrating on support facilities. It is about three miles outside town, in the midst of quiet fields.

Downtown, a few walls are scrawled with slogans: "The base of death must not be built" or "Out with the Americans and their missiles." But these, and a scattering of circled anarchist A's, including one on the forehead of the stone Diana in front of City Hall, give little indication that tempers are running high.

Last year, according to local pacifist leaders, 12,800, almost half the town's population, signed a petition against the base. And although many townspeople questioned on the issue appeared reticent or even indifferent, others were clearly divided.

"Why can't they build it somewhere else? It's a danger to us all," said a gray-haired man who refused to give his name. "It will bring no good, only evil," said Rosario Davola, an elderly man on his way home with a large bag filled with garlic.

"The missiles will bring work to Comiso," said the owner of a small restaurant, who said his name was Biaggio. "Of course, this could backfire if rents and other prices started to rise."

"They're saying the Mafia will come in, but Comiso is a clean town and it won't be that easy to turn it dirty," said Filippo Incardona, a painter.

Last month, Italy's high commissioner for anti-Mafia activities, Palermo prefect Emanuele de Francesco, said in an interview with an Italian daily that "certain Italian families" had bought up thousands of acres of land in Comiso and nearby towns.

Last week, magistrates and high-ranking police officials in Palermo confirmed that some of the families involved were Mafia suspects and that while many of the purchases went back several years, others had taken place since construction began on the base in April 1982.

Land investment by the wealthy is traditional in Sicily, but such investment has become particularly appealing as the need to launder proceeds from the enormous Palermo-based narcotics trade has grown, investigators have said.

Although the Communists in Sicily have long expressed concern about Mafia exploitation in Comiso, local opponents of the base in Sicily, where 1.2 million protest signatures were collected last year, have generally stressed the danger of nuclear war.

Earlier this month, Communist leader Enrico Berlinguer announced that demands to halt construction at the base while U.S.-Soviet negotiations are in progress will be a major plank in the platform for next month's parliamentary elections.

But in Comiso, where a new city council is also to be elected, pacifists, Catholic leftists, Communists and various far-left political groups are focusing on concern that the presence here of an American military base will have a disruptive effect on the small, tradition-bound rural community.

"Experiences elsewhere teach us that the installment of an American military base could destroy local customs and distort the economy," said Antonio Iurato, an organizer at the Unitary Committee for Disarmament and Peace that has organized several large demonstrations here.

"The case will be a major issue, although not the only one, in our campaign," said local Communist leader Salvatore Zago. "We can't promise that if we do well, the base will be closed. Such decisions are made elsewhere," he acknowledged.

The Italian government approved the construction of an intermediate-range missile base in December 1979 and successive coalitions of Christian Democrats, Socialists and smaller centrist parties have never wavered in their commitment.

Diplomats have said Sicily was chosen because of its southern Mediterranean geopolitical location and Comiso because of the old airport and the sparse population that lends itself easily to plans to disperse the missiles into the countryside by truck in the event of an attack. Little importance appears to have been given to the fact that Comiso and nearby towns are traditional Communist strongholds.

Although this year's election results will thus clearly represent a test of local opinion, Italian and U.S. authorities do not appear overly concerned. "There are always problems when you put a base in a small town anywhere," a U.S. officer said recently, "but in time, generally because of the economics involved, these things tend to work themselves out."