Sen. George McGovern (D-S.D.) yesterday released a denunciation of Zbigniew Brzezinski, President Carter's national security affairs adviser, while Senate Majority Leader Robert C. Byrd (D-W. Va.) gave a tough speech on foreign affairs implicity supporting Brzezinski's recent statement.

The two senators differing reactions to recent tensions in Soviet-American relations suggest the intensification of the debate on foreign policy that has been evolving recently.

McGovern's statement, which an aide said the senator wrote personally agter watching Brzezinski on NBC's "Meet The Press" Sunday, was an unusually harsh attack on a key presidential aide.

McGovern ridiculed Brzezinski's decision to give Chinese officials detailed secret briefings on Soviet-American arms talk on his recent visit to Peking, and accused him of spreading unwarranted alarmism about the state of the world.

"No national security adviser . . . wants to appear fainthearted in the face of a genuine crisis," McGovern said. "But to avoid being 'chicken Lit-

"We cannot conduct foreign policy as though every stirring in Africa, Asia or the INdian Ocean is another Cuban missile crisis," McGovern added.

In his "Meet the Press" appearence, Brzezinski accused the Soviet Union of violating "the code of detente," and painted a bleak picture of communist ambitions in Africa.

McGOvern accused Brzezinski of exaggerating the danger posed by the Soviet advisers and 30,000 Cubans in Africa - "a vast continent of 300 million people" - and said there was no reason to think Russians could be more effective than other white-skinned Europeans in any effort to sustain colonization in that continent.

Reciting a list of U.S. assets and claiming that America is "the preminent power in the Western Hemisphere, in the NATO European partnership, in the Mediterranean, in the Middle East and in the Pacific," McGovern said. "It does not serve our interests to elevate Soviet influence and power while minimizing our own."

McGovern said the United States should pursue detente, which he define as an area of Soviet and American mutual interests that cover "a few areas such as arms control to avoid mutual extinction" and retard nuclear proliferation, without feeling the country had to also "love the Russians."

He said he thought this view was shared by Secretary of State Cyrus R. Vance, chief arms control negotiator Paul Warnke and U.N. Ambassador Andrew Young. "They are not, apparently, the views of Mr. Brzezinski," McGovern said.

"It would appear that as of this moment we do not have a coherent foreign policy, but a collection of conflicting voices," McGovern said. "I pray that in the contest for the mind of the president, calm and common sense will prevail over the strategy of crisis and confrontation."

Byrd, meanwhile, speaking to the Delta Coincil in Cleveland, Miss., said the Soviet Union had raised serious doubts, by its recent behavior, about its willingness to adhere to a new strategic arms treaty.

Byrd called on the Carter administration to proceed at once with construction and deployment of enhanced-radiation warheads - the so-called neutron arms - unless the Soviets offer a quid pro quo in return for U.S. abstention from building the weapons.

Despite detente, Byrd said, recent Soviet policies in Africa indicate "that the Soviet Union has not swerved from its commitments to foment chaos wherever it believes it can benefit."