Zbigniew Brzezinski, who worked side by side with Vice-President Walter F. Mondale during the Carter administration, is disclosing his differences with Mr. Mondale, then and now.
President Carter's national security adviser is particularly critical of Mondale for backing a freeze on nuclear weapons. Talking to reporters over breakfast the other morning, Brzezinski said: ''On the freeze I disagree with Fritz. I hope this is only a temporary accommodation with what probably is the predominant viewpoint in the Democratic Party.
''The freeze is a hoax being perpetrated on well-meaning people, because nobody knows what the freeze is. Nobody knows how it is to be achieved. Nobody knows how it is to be implemented. The freeze is essentially like passing a resolution saying, 'We are in favor of eliminating poverty.' But if it is not specified how it is to be done in a manner that at least approximates the possibility of implementation, then it is a hoax.''
Brzezinski, author of recently published memoirs ''Power & Principle,'' won't break with Mondale. But he comes close. He says that if between now and the fall of 1984 Mondale shows he is able to shape a bipartisan foreign policy, ''then I'll support him.''
''But,'' he adds, ''if I have the feeling that on defense issues, on security issues, and on the Middle East, he takes an essentially partisan position, calculated more for domestic benefit, and surrounds himself with advisers who represent the left of the Democratic Party spectrum, then I will not support him.''
Here Brzezinski added a caveat: ''Unless his opponent is extreme to the right. And then the choice becomes very painful, reminiscent of 1972 (the Nixon-McGovern race).''
Brzezinski underscored the need to return to bipartisanship if a president is to be successful in foreign policy. When asked if he was more comfortable with the foreign policy of Ohio Democrat Sen. John Glenn than with that of Mondale, he replied: ''The two people on both sides of the political fence who are best positioned to reach a bipartisan foreign policy are (Tennessee GOP Sen. Howard H.) Baker and (Senator) Glenn, for a variety of reasons related to their past positions or careers. But this doesn't mean that Mondale couldn't do it. Or that Bush couldn't do it.''
Brzezinski doesn't speak for any political group or faction. But he's a relative hard-liner who commands considerable credibility among conservative Democrats, the Southern Democratic constituency in particular.
Thus, Brzezinski's position, added to Carter's recent unhappiness over criticism from Mondale, adds to the Democratic frontrunner's mounting problem: As he tries to capture the liberal vote, how is he going to keep the Carter people and their sizable following behind him?