IT is easy to sympathize with President Reagan's call in his speech at Georgetown University for a more bipartisan approach to foreign policy. The President did no more than echo a number of his predecessors when he urged a return to ''Amerca's honorable tradition of partisan politics stopping at the water's edge, Republicans and Democrats standing united in patriotism, and speaking with one voice.''
However one may sympathize, bipartisanship has been the exception rather than the rule in American politics. In the country's formative years there was nothing sacrosanct about foreign policy. The New England states were so upset by the War of 1812 that they seriously considered leaving the Union. The war with Mexico in 1845 was widely denounced as an ''unjust'' war, and an obscure Illinois congressman named Abraham Lincoln challenged the government to provide evidence of Mexican aggression.
For the most part, presidential elections have focused on domestic rather than foreign-policy issues. But when international issues arose, no one pulled any punches. In 1900 the Democrats chose to soft-pedal a variety of domestic issues and base their campaign on opposition to the annexation of the Philippines. They lost. Sometimes it was the incumbent party that campaigned on foreign policy, as when Woodrow Wilson secured reelection running on the slogan, ''He kept us out of war.''
The origins of bipartisan foreign policy are probably to be found in the period just before and during World War II, when there was a widespread consensus on foreign-policy issues. During the war, the Republicans sent John Foster Dulles to Washington to act as a foreign-affairs liaison with the Roosevelt administration.
But the ideal of bipartisanship probably reached its fullest flowering with the ''conversion'' of Arthur Vandenberg. In the period prior to World War II, the Michigan senator had been one of the most conspicuous isolationists. But he saw Pearl Harbor as bringing an end to isolationism, and in the postwar years his position on the Foreign Relations Committee brought him into daily contact with the problems of European reconstruction.
Vandenberg chaired the Foreign Relations Committee in 1947 when President Truman was sounding out sentiment in Congress concerning his program of aid for Greece and Turkey. Dean Acheson later described how Truman briefed Vandenberg on political trends in Europe, characterizing the unrest in Greece and Turkey as largely Soviet-inspired. ''I can see Senator Vandenberg now,'' Acheson wrote, ''suddenly leaning forward on the sofa in the President's office and saying, 'If you will say that to the whole country, I will support you!' ''
This was the high-water mark of modern-day bipartisanship, but it was short-lived. Cooperation was strained by another election year. Bipartisanship deteriorated further amid an unpopular and inconclusive war in Korea, the controversial firing of Gen. Douglas MacArthur, and the rise of Sen. Joe McCarthy.
On a tactical level, there will always be some cooperation between the principal parties on matters relating to foreign policy. Because Congress must approve the appropriations that support administration activities abroad, the appropriations process requires some degree of give and take. But those who lack the votes, say, to block aid to El Salvador, feel no obligation to support administration policy there.
Genuine bipartisanship is unlikely to reappear in the absence of a blatant attack on a recognized US ally - preferably one with a good lobby on Capitol Hill.
The basic reason for this discouraging forecast is not that politicians are more opportunistic today, or that elections are more oriented toward foreign-policy issues. The one trend that virtually assures that politics will not stop at the water's edge is that Congress now regards as its prerogative the foreign-policy direction that was once the exclusive domain of the executive branch.
In his speech, President Reagan complained of ''a rash of congressional initiatives to limit the president's authority in the areas of trade, human rights, arms sales, foreign assistance, intelligence operations, and the dispatch of troops in times of crisis.'' As long as these continue, bipartisanship in foreign affairs will prove as elusive as it was in 1812.