IN theory, politics stop at the water's edge. In practice in the United States the theory almost never works. Rare is the politician who would pass up a chance to win votes on a foreign-policy issue. Ronald Reagan is no exception. In 1976 he used arms-limitation policy (SALT I) against Republican President Gerald Ford during the Republican preliminaries. In 1980 he pounded President Carter over the Panama Canal Treaty.
It's different now that Ronald Reagan is himself President. Now he wants his opponents to ''stop at the water's edge'' and give him full support for his foreign policies. Few ever preached a more eloquent sermon for bipartisanship in foreign policy than Mr. Reagan did last week. Few have used that appeal more effectively as a partisan weapon.
There is a peculiarly serious problem about politics and foreign policy in the United States because so many Americans have foreign origins, hence care about what happens elsewhere. Americans of Polish background want the US to get the Soviets out of Poland. Irish Americans want Washington to get the English out of Ulster. Black Americans want the US to force South Africa to abandon apartheid.
Former Vice-President Walter Mondale and Sen. Gary Hart used a foreign-policy issue (the location of the US Embassy in Israel) so vigorously against each other in the New York primaries that it began to worry the community of senior experts in the foreign-policy area. A former White House aide, Douglass Cater, discussed the problem with former Secretary of State Henry Kissinger. They drafted a statement. It said: ''While mindful that robust argument has always accompanied the choosing of the US president, we caution that excess of rhetoric can have lasting impact on the conduct of foreign policy, causing risk for this nation which has been called on to play the role of leader in the thermonuclear age.''
The statement was signed by former Presidents Gerald Ford and Jimmy Carter and by six former secretaries of state: Dean Rusk, William P. Rogers, Henry Kissinger, Cyrus R. Vance, Edmund Muskie, and Alexander M. Haig Jr.
Mr. Reagan is an example of how campaign rhetoric can have ''lasting impact on the conduct of foreign policy.'' During the 1980 campaign he criticized China policy and gave the impression that if elected he would revert to recognition of the emigre regime on Taiwan as being the true government of China.
In office Mr. Reagan renewed the sale of American weapons to Taiwan over vigorous protests from Peking. Shortly thereafter (October 1982) Peking opened talks with Moscow. The fourth round in those talks was held in Moscow during the last two weeks of March. Formal diplomatic relations between China and the USSR were broken nearly 20 years ago and have not yet been restored. They keep talking about it.
Mr. Reagan claimed in his speech at Georgetown University on April 6 that in his foreign-policy actions ''we have throughout sought to revive the spirit that was once the hallmark of our postwar foreign policy - bipartisan cooperation between the executive and legislative branches of our government.''
Remarkable and effective bipartisanship in US foreign policy existed for the first 15 years after World War II in respect to Europe. Republicans backed Democrat Harry Truman in the economic recovery of Europe and in forming the NATO alliance, but attacked him mightily for allegedly ''losing China.'' Democrats largely supported President Eisenhower on foreign policy.
Mr. Reagan sought a bipartisan position on nuclear weapons and Central America by setting up bipartisan commissions. So far it has worked for nuclear weapons. It is working over Central America, after a fashion. Democrats think Mr. Reagan is overusing the military commitment which the Democrats reluctantly accepted in the Kissinger commission report.
There has been no Middle East policy agreed on in advance. Mr. Reagan in his speech accused Congress of being responsible for failure in Lebanon. Democrats are heatedly accusing him back. The key political question is who will be blamed if El Salvador collapses.