With two days to go before this Central American nation votes in an election that the Reagan administration has described as crucial for the future of democracy here, a tense and uncertain calm settled across capital and countryside in El Salvador today.

Faced with a call from El Salvador's leftist guerrillas to stay away from the polls and an announced determination by the Army to put down any offensive the guerrillas might start to spoil the elections, many Salvadorans appeared today to have decided to stay out of harm's way. The Litoral, the second largest highway in the nation, was virtually deserted, and municipal officials contacted by telephone reported that there was almost no traffic coming into the principal towns.

In the capital of San Salvador, foreigners provided much of what activity there was touching on the elections. As the U.S. government continued to position itself for the post-election period, Ambassador Deane R. Hinton told reporters that Christian Democratic politician Jose Napoleon Duarte, current head of the junta and of the party widely perceived here as having official American backing, "is not confident of winning these elections."

Hinton also voiced conciliatory remarks about former major Roberto D'Aubuisson, an extreme rightist who appears to be the strongest challenger Duarte faces. The U.S. envoy said if D'Aubuisson's party wins, "he should be judged on his future actions and performance" and not on the widespread allegations that he has directed political murders and other human rights abuses here.

At El Salvador's International Airport, a blue-white-and-gold jet with the American Flag and "United States of America" on its side brought American dignitaries to join more than 100 other official observers of the vote, which is for a 60-member assembly to write a new constitution for this country.

"We hope the elections will prove a first step on the road to political stability in El Salvador," Sen. Nancy Landon Kassebaum (R-Kan.) said before driving in a heavily protected motorcade to the quiet capital. Refusing to answer reporters' questions, Kassebaum said, "We are here to witness and report the fairness of the election, not render judgments on the outcome."

In San Salvador, campaign posters abound and traffic clogs the streets; stores are open and the major military presence of two days ago appears dramatically reduced.

But here in the countryside, a walk down a road is filled with the kind of tension that suggests why people were not moving outside their immediate neighborhoods today.

Silent and half hidden, a small squad of soldiers crouched nervously in a ditch beside the road leading into San Marcos de Lempa this morning. They looked out over burned-off brush and leveled trees toward a forest used by guerrillas as a regular base of operations.

By early afternoon the guerrillas' clandestine Radio Venceremos announced, after days of relatively little activity, "We are tightening the circles around the enemy." They announced pushes near San Miguel, against small towns south of Usulutan and efforts to disrupt traffic even near the relatively peaceful town of Santa Ana.

A journalist reported seeing a group of guerrillas a half mile from the outskirts of the provincial capital of Usulutan late in the day, and unconfirmed reports circulated here that the guerrillas had begun to cut off roads leading into a number of towns in a coordinated effort.

Hinton's remarks, made to wire service reporters at an invitation-only press conference last night, seemed also to reflect the new level of uncertainty that has developed in the campaign's closing days, as D'Aubuisson's campaign picked up momentum and reports of new initiatives for negotiations between the Reagan administration and the governments of Nicaragua and Cuba were being circulated.

Hinton warned that the calm might be illusory because guerrilla arms supplies are at an "all-time high, and you'll probably be hearing or seeing them in the next few days."

The elections appeared certain, when they were first announced last year, to endorse and legitimize the Christian Democratic-military coalition the United States has backed here as a centrist alternative to left and right extremists since 1979.

But a new party, The National Republican Alliance, has emerged under the leadership of D'Aubuisson, a man the U.S. Embassy has frequently denounced during the past two years as an ultra-rightist terrorist over the past two years, as a possible decisive factor.

After reporting that Duarte was not now predicting victory, Hinton declined to speculate about who would win. But he did say that "D'Aubuisson is a leader. There's no doubt about it, that he has that charismatic factor. He comes across in the caudillo populist strongman tradition of Latin America."

Hinton added that while D'Aubuisson in the government might make it more difficult to get an already reluctant U.S. Congress to approve more aid, "we must not prejudge him. He denies the rough stuff. He should be judged on his future actions and performance and not. . .on his past performance."