''AS I see it, members of Congress, and particularly members of the Senate, have a constitutional obligation to reexamine constantly and discuss the foreign policy of the United States. If we permit appeals to unity to bring an end to that criticism, we endanger not only the constitutional liberties of the country , but even its future existence.''
That is not Speaker Thomas P. O'Neill talking about President Reagan's foreign policy in 1984. It is, rather, Senator Taft of Ohio - Mr. Republican - talking about President Truman's foreign policy in 1951.
Taft's remarks are worth recalling because a Republican President is trying to stifle debate over his foreign policy by appeals to bipartisanship. Reagan is running for reelection by running against Congress.
Truman also ran against Congress in 1948. But Truman's quarrels with the ''do-nothing Republican 80th Congress,'' as he labeled it, were over domestic issues. Under the leadership of Republican Sen. Arthur Vandenberg, the 80th Congress had given the Truman foreign policy solid support, among other things authorizing the Marshall Plan and laying the groundwork for NATO.
Vandenberg is the father of the idea of bipartisanship, but he preferred the term ''nonpartisanship.'' This means an approach to issues of foreign policy based on the national interests of the United States rather than the political interests of the Democratic or Republican Party. Vandenberg also made the observation that if the administration wants the other party in on the crash landing, it had better include the other party in the takeoff.
To presidents, bipartisanship means support of their policies - not at all the same thing. These policies are frequently airborne, or racing down the runway, before the other party is consulted. Once a policy is in effect, presidents tend to think that debate is weakening.
Lyndon Johnson deplored criticism of the Vietnam war because, he said, it encouraged the Viet Cong and the North Vietnamese. What was required to make his policy successful was a show of national unity and determination. That was precisely what was wrong with the policy: There was no national unity and determination.
President Reagan is arguing that Congress ought to support his policy in Central America, as it should have done, but did not, in Lebanon. Again, one of the things wrong with these policies is the lack of national unity.
Some in Congress criticized the Lebanon policy, but Congress as a whole supported it. It was Reagan who sent the Marines and it was Reagan who pulled them out. Congress had authorized their presence for a longer period. However, Secretary of State George Shultz says that debate over the question ''totally took the rug out from under our diplomatic effort.''
The administration is making the same points with respect to Central America, where, the President has said, ''We face one of the major challenges for democracy in our time.'' He added, ''Debate on this issue has strayed too far from reality.''
Presidents arrive in the White House through the working of the United States political system, then forget the principles of that system. One principle is that sound policy requires uninhibited public debate. Another is that policy ought to show respect for the opinion of mankind.
Reagan not only wants Congress to give him a free hand in Central America, but he no longer cares what the rest of the world thinks. His administration has said that it will pay no attention to the World Court. His ambassador to the UN scorns a ''legalistic approach to international affairs.'' His deputy secretary of state says mining harbors in Nicaragua is self-defense, as though it were Nicaragua invading the US. The UN Security Council voted 13 to 1 to condemn this , the one being the US.
Reagan has demonstrated an uncanny ability to fool people by saying one thing while doing the opposite. He may get away with it again. But the country deserves a major debate. That is one reason we have a Congress.