wo U.S. senators who came here six days ago to sound out possibilities for easing Soviet-American tensions said today they are returning home discouraged by the magnitude of the problems separating the two countries.

Sens. Charles McC. Mathias (R-Md.) and Alan Cranston (D-Calif.) said their talks with top political and military leaders left them with the impression that the current drift in relations was likely to continue for some time.

At a news conference before their departure, both men said they were disturbed by the gap in mutual perceptions with both sides making "worst case assumptions."

This, they suggested, has produced a climate that may not threaten a major war but certainly endangers whatever chances there may be for making a better peace.

Both said they would report to Secretary of State Alexander M. Haig Jr. on their conversations with Foreign Minister Andrei Gromyko, Marshal Nikolai Ogarkov, chief of staff of the armed forces, and other Soviet officials.

"I'll leave Moscow without optimism but not without hope," Mathias said without explaining the reason for his continuing hope.

Cranston said he found it "discouraging" that Soviet officials blamed the United States for the current tension.

"I was unable," he said, "to get from any Soviet person with whom we met any acknowledgment of any specific action the Soviet Union has committed that contributed to the decline in our relations."

The senators quoted Ogarkov as saying that the Soviet Union was seeking only strategic parity not superiority. The Soviet officials said they were concerned about U.S. actions that suggested to them that the United States was reaching for military superiority.

Ogarkov was also quoted as saying that neither side could win a nuclear war and that the Soviet military leaders have ruled out the possibility of a limited nuclear exchange. Once a nuclear weapon is used, he was quoted as saying, there would be no way to limit any East-West conflict.

Since Mathias and Cranston, who were both supporters of the SALT II treaty, came here without a mandate from the Reagan administration, it was not likely that they would be told anything but the official line on these matters.

This is particularly true because Gromyko and Haig are due to hold their first meeting since Haig took office in New York later this month. In the past such meetings were used to advance private assessments of each other's presumed ambitions and interests, and it was unlikely that the Soviets would disclose to the traveling senators any possible points of compromise.

The senators said, however, that they had not detected any signs of Soviet flexibility on such questions as Afghanistan, emigration and the deployment of SS20 intermediate-range ballistic missiles in the European part of the Soviet Union.

They said they told their Soviet hosts that the invasion of Afghanistan had effectively killed chances for the ratification of the SALT II agreement.

One senior Central Committee official, Valentin Fallin, was quoted as retorting that the Carter administration had decided for political reasons not to push for the treaty's ratification "on Dec. 6, 1979," which was more than three weeks before the invasion of Afghanistan.

Cranston, the Democratic whip in the Senate who was supposed to guide the treaty through that chamber, reportedly dismissed Fallin's claim as inaccurate.

Cranston and Mathias, both members of the Senate Foreign Relations Committee, are the first senators to visit here since Ronald Reagan became president. Committee Chairman Charles Percy (R-Ill.), visited here before Inauguration Day.

Asked if the Polish crisis came up in the talks, Mathias said he and Cranston had told the Soviets that "any kind of military action in Poland would have a very sharp reaction in the West, perhaps greater than Afghanistan." He did not report Soviet responses to this.

Mathias said his lack of optimism was grounded in what he saw as the "political realities in the Soviet Union and the United States."

A resolution of outstanding issues between the two countries, such as Afghanistan and the use of Cuban proxies in Africa, is not likely, he said, "because the political realities of the Soviet Union render necessary changes difficult."