Politics can never stop "at the water's edge" between the Reagan administration and its opponents in Congress.

The gulf between them is not limited to such seemingly disconnected issues as Nicaragua, nuclear arms control, human rights, foreign aid, contributions to multinational organizations and other inflammatory international subjects. These clashes illustrate disagreement about U.S. security interests in the broadest sense.

It is not surprising that this discord brought angry polemics in the Republican and Democratic platforms and in the party conventions.

The Reagan administration attempted to bridge part of the foreign-policy divide with commissions on Central America, headed by Henry A. Kissinger, and on strategic forces, chaired by Brent Scowcroft.

The administration anxiously sought Rep. Michael D. Barnes (D-Md.), chairman of the House Foreign Affairs subcommittee on Western Hemisphere affairs, to serve with the Kissinger group. He called the commission "a very useful process" that "elevated" the debate but said he still sees U.S. support of the "contras" fighting Nicaragua's Sandinista regime as "disastrous."

On El Salvador, the administration has been more successful, marshaling congressional support for President Jose Napoleon Duarte on his pledges for democratic rule of that war-battered nation.

The Scowcroft commission's work on the MX missile also won some bipartisan backing when the administration made strategy concessions at a sensitive stage. That limited accord, coupled with the Soviet walkout from two sets of nuclear negotiations in Geneva, shifted part of the attack on the administration's policy to the Soviet Union.

The MX history can be traced to "the tragic failure of congressional efforts in 1969-70" to head off the Nixon administration's development of multiple-warhead missiles, according to Alton Frye of the Council on Foreign Relations.

Reviewing that history in a study of executive-legislative relations by the Center for Strategic and International Studies, Frye reported: "Congress has reflected, articulated and refined ideas which . . . have often seemed wiser as matters unfolded."

Congress, of course, has not been unified on these issues, but neither have recent administrations. The Reagan administration has experienced much internal discord on foreign policy in the State-Defense-White-House triangle.

The administration has learned that, when Congress "senses mismanagement, they move in quickly," said Brian Atwood, a former assistant secretary of state for congressional affairs and now executive director of the Democratic Senatorial Campaign Committee. "It happened to us in the Carter administration."

On the other hand, members who operate across the executive-legislative divide, such as Rep. Les Aspin (D-Wis.), who has worked with the Scowcroft commission as a broker for nuclear-weapons compromises, become vulnerable to crossfire. Sarcastic "Reagan and Aspin '84" buttons circulated this summer at the Wisconsin Democratic convention.

"I always thought what we really needed," Aspin said, "was a Democratic administration to negotiate these things, and then a Republican administration take office and get the thing approved. But Ronald Reagan wouldn't follow the script. He wouldn't go with SALT II," the unratified U.S.-Soviet arms accord negotiated by President Carter.

With neither the president nor Congress able to impose its will clearly in foreign policy, champions of presidential supremacy were elated when the Supreme Court ruled in June 1983 that the House acted unconstitutionally in a minor immigration case. It was initially hailed as a shattering blow to Congress' widespread resort to "legislative veto" power, through which one chamber or both could block specific actions by the executive branch.

Like other supposedly clear-cut divisions in authority, that expectation has faded. The impact on the War Powers Resolution and scores of other laws will take years to sort out. Congress will find other avenues for assertiveness.

"The result," according to I.M. Destler of the Institute for International Economics, "will most likely be a Congress weaker on the large issues and more intrusive on the small -- the opposite of what effective foreign policy-making requires."