Late last week, Senate Majority Leader Robert J. Dole (R-Kan.) wrote a classified letter to Secretary of State George P. Shultz attempting to secretly clarify one of those nuances of foreign policy that leaders are often loath to put in writing.

The letter, a copy of which was made available to The Washington Post, demonstrates the intricate pulling and tugging between the Senate's conservative wing and the Reagan administration over the most effective way to support the anticommunist "freedom fighters" President Reagan has identified in a growing list of Third World countries.

The administration last month notified the House and Senate intelligence oversight committees that it plans to give covert assistance to Angolan guerrilla leader Jonas Savimbi, who last week completed a tour of Washington and New York to seek U.S. support for his fight against the Marxist government of Angola. The notification was by secret presidential "finding" under the National Security Act.

In late January, Shultz asked Senate leaders to give the administration an expression of support so the Central Intelligence Agency could proceed with the covert aid. Dole obliged by drafting a resolution urging U.S. support for Savimbi.

No such expression of support is required by law, but shrewd political instincts persuaded Reagan's national security advisers of the need for what some call "political cover" before they involved the United States in another CIA-backed paramilitary operation against a foreign power.

Despite the appearance of support for Savimbi in the upper reaches of the administration, a number of Savimbi's strongest supporters in the Senate suspected that other members of the administration would like to derail the covert operation, and last week they expressed their doubts to Dole.

Their chief concern was that the administration would lend only moral support or some form of military assistance that fell short of Savimbi's critical needs: antitank and antiaircraft weapons to fight Soviet armor.

At a private lunch with Savimbi Feb. 5, Dole and a group of senators, most of them conservatives, developed a strategy to lock the administration into a commitment to give Savimbi what he says he needs.

The following letter, classified "secret" by Dole's staff because it implicitly refers to the pending covert operation, according to Republican Senate sources, is a gambit that is believed to have paid off for Savimbi.

Republican sources said this week that the White House and State Department are considering redrafting the president's secret "finding" for transmission to the Senate, but this time the presidential document would specifically call for providing Savimbi with lethal weapons to combat Soviet-built tanks and to shoot down Soviet-built assault helicopters.

The letter was signed by senators Dole, John Heinz (R-Pa.), Steve Symms (R-Idaho), William V. Roth Jr. (R-Del.), Mitch McConnell (R-Ky.), Orrin G. Hatch (R-Utah), Robert W. Kasten Jr. (R-Wis.), Pete Wilson (R-Calif.), Jeremiah Denton (R-Ala.), James A. McClure (R-Idaho) and Phil Gramm (R-Texas).