The landslide election of Ronald Reagan and a Senate dominated by conservative Republicans abrutply ushers in a new era for Soviet-American relations after years of bilateral attempts to find common ground for cooperation under the rubric of "detente."

The official Soviet reaction today to what one Western diplomatic source called "very sobering" election results for the Kremlin was to blink and view the vote as a resounding defeat for President Carter's anti-Soviet policies.

But neither the media nor Soviet officials in private contacts faced the fact that the election turned out of office a number of Democratic senators whose votes were crucial to ratification of the long-delayed SALT II treaty.

The Soviets long have called the treaty, and the complex negotiating process for limiting both sides' strategic nuclear arsenals, the cornerstone of "relaxation of tensions" between East and West.

Both the Soviet commentators and officials in analyzing the election results ignored the fact that Carter in his final weeks of campaigning had called the election a referendum of the SALT treaty, which the president shelved for ratification after the Soviet invasion of Afghanistan last December. Reagan has said he would scrap the treat and seek to renegotiate it.

Soviet President Leonid Brezhnev months ago said the treaty, which he and Carter signed at their June 1979 summit in Vienna, cannot be renegotiated. Foreign Minister Andrei Gromyko in later declarations reasserted the Soviet position that the Senate must ratify the agreement as signed by the two heads of state.

One senior Soviet official told correspondents today that Reagan may bring a "more businesslike, but tougher" approach to bilateral relations. But his remarks fell well short of serious analysis of the sudden problem of the nuclear weapons treaty. The Kremlin in March 1977 bitterly rejected an attempt by then secretary of state Cyrus Vance on a special mission to Moscow to reopen the basic tenets of the treaty and seek instead an overall agreement for general reductions an nuclear striking power.

The SALT II treaty is valued by Moscow because it confers strategic parity on the Soviets, who have devoted enormous economic resources during Brezhnev's 16-year tenure to building up their strategic, as well as conventional, forces.

Confronted by the unmistakably conservative swing in the U.S. electorate yesterday, the official Tass press agency described the outcome as a victory for detente. It laid Carter's defeat on "hundreds" of broken promises during his four years, but emphasized that "the greatest damage was inflicted by [his] turn in foreign policy from the course toward detente to the course toward heightening international tensions and dangerous doctrines in the use of nuclear weapons."

Tass said that "the voters rejected the provocative stand in respect to detente, demonstrating their understanding of the irrefutable fact that not a single question can now be resolved along the lines of the arms race."

The official version of Carter's defeat hewed to the Soviet portrait drawn through months of bitter denunciation of Carter as a man who abandoned detente and sold out to the military-industrial complex more than a year ago as a way of winning reelection. The only other recent U.S. president to have been attacked so personnally was Lyndon Johnson during the Vietnam War.

The Tass dispatch from New York, the only substantive official comment on the election, offered an olive branch to Reagan, in line with what one senior Soviet Journalist today asserted is "traditional willingness to work with any U.s. president." Tass said the Kremlin "has always come out in favor" of Soviet-American relations "on the basis of the principles of peaceful coexistence" as set forth at the 1972 Nixon-Brezhnev summit.

Soviet sources today repeatedly asserted the belief that Reagan, like Nixon, would move from tough anti-communist campaigning to exploring new cooperation with Moscow. Foreign analysts here see the Kremlin taking satisfaction from Reagan's obvious coolness toward China, Moscow's archenemy and ideological rival. They also see the Soviets intent on exploiting strain between Washington and its West European allies to restrain any Reagan impulses to direct confrontation with Moscow.

But the election has wrought such changes to the basic political structure and shadings in the next Congress and administration in Washington that analysts forsee a long period of dormancy for the Kremlin in taking any fundamental bilateral intiatives until well after Reagan takes office.