A senior member of the Soviet leaderhip, Konstantin Chernenko, sharply assailed the Reagan administration today as a group of "dangerous" politicians whose "bellicose" policies could return the world to the days of the Cold War.

Chernenko, 71, a Politburo member and the closest adviser to President Leonid Brezhnev, hinted broadly that Moscow was increasingly wary about prospects for an improvement in its relations with Washington and that it may place them in the deep freeze for the rest of President Reagan's term.

Today's speech -- coming on the heels of one by Brezhnev Wednesday -- appeared to foreshadow a more aggresive Soviet propaganda line against the United States.

Brezhnev, in his speech before the entire command structure of the Soviet armed forces, accused the Reagan administration of "adventurism, rudeness and undisguised egoism."

The attacks on the United States are expected to provide the basic rationale for the Kremlin's increased military budget and accompanying need for belt-tightening throughout Soviet society.

The slight shift in Moscow's position -- Brezhnev, while defending his "peace policy" noted that "new questions" had arisen and had to be quickly solved -- is believed to stem from pressure by the armed forces. There have been indications during the past year that the military chiefs were dissatisfied with the apparent immobility here and with what they obviously regard as an inadequate Soviet response to the American challenge.

Chernenko today angrily charged that Reagan's term in office was marked by failures both at home and abroad and referred to huge budget deficits, high unemployment and the rising numbers of business bankruptcies in the United States.

He repeated Moscow's willingness to arrest "a further growth of tensions in Soviet-American relations."

"We stand for their normalization and improvement," he said, "and we are prepared to engage in businesslike and detailed negotiations which must, of necessity, take into account the interests of both sides.

"If, however, Washington proves unable to rise above primitive anticommunism, if it persists in its policy of threats and diktat, well then, we are sufficiently strong, and we can wait. Neither sanctions nor bellicose posturing can frighten us."

The remarks made at a political rally in Tbilisi, the capital of Soviet Georgia, appeared to be a somewhat more strident echo of Brezhnev's speech.

The Soviet leader clearly forecast a new wave of weapons modernization to counter an "unprecedented" U.S. arms buildup as he asserted that the country would spare "nothing" to keep its military forces "up to mark." He specifically talked about modern military technology.

The highly unusual conference between the top political and military leadership indicated that a major reassessment of Soviet security policy is underway and that the coming meeting of the Soviet Central Committee is likely to approve sharp increases in the military budget for the next year.

It is said here in well-informed circles that the U.S. congressional elections could play a decisive role in Moscow's policy toward the United States for the next two years.

In this view, marginal Republican losses, especially in House races, are expected to produce an even more belligerent U.S. attitude toward the Soviet Union and practically foreclose all chances for repairing relations with the Reagan White House.

On the other hand, substantial Democratic gains are seen as likely to have a restraining impact on the administration. This, in turn, would not necessitate a radical turn in Kremlin policies, according to sources.

Whatever Soviet calculations, it seems that the Russians are conducting their reassessment on the "worst case" basis. Chernenko, in portions of the speech distributed by the news agency Tass, hinted at that.

"For almost two years the rulers of the United States have been flexing their muscle. For almost two years abusive language aimed at the Soviet Union and other socialist states has been heard from Washington. For almost two years myths about a Soviet threat and the 'hand of Moscow' have been serving as a kind of ideological foundation of U.S. foreign policy," he said.

In an indication that his was not a policy statement, he quoted an unnamed American newspaper to substantiate his argument that Reagan's policies have damaged U.S. relations with Western Europe, Japan and Latin America.

"If Moscow could manage to infiltrate its agents into the White House," he quoted the newspaper as saying, "it could still hardly do more to undermine the authority of America than this is being done by the present administration."

Like Brezhnev, Chernenko also drew a clear link between the deteriorating Soviet-American relations and Moscow's hope for a rapprochement with China. But Chernenko avoided any negative references to China and referred to it as "the great Chinese neighbor," a phrase far warmer than anything heard from a top Kremlin official in many years.