President Reagan, describing last week's congressional defeat of new U.S. funding for the Nicaraguan contras as "not the final word," promised last night to return to Congress and fight for more funds for them.

"Get ready . . . the curtain hasn't fallen, the drama continues," the president said to applause at a dinner hosted by a conservative political group.

Reagan, in some of the strongest language he has used since the 219-to-211 House defeat of his contra-aid package Feb. 3, said he would have won "if we could have turned around four or five votes."

"So, let me make this pledge to you tonight: We're not giving up on those who are fighting for their freedom -- and they aren't giving up either," he said at the annual dinner of the Conservative Political Action Conference, an umbrella organization for some of the most influential conservative groups in Washington. It has long provided a receptive forum for the rebels, who, Reagan told the group in 1985, are "the moral equivalent of the Founding Fathers."

Last night the president returned to that theme in a speech in which he also sought to dissuade conservatives from believing their movement has grown tired and leaderless. He pledged anew to campaign hard for whomever the Republican Party nominates for president, saying that he wants the American people "to replenish our mandate."

Reagan ticked off administration economic successes but sounded less optimistic when discussing the contra issue. He called the House vote "a setback to the national-security interests of the United States and a sad moment for the cause of peace and freedom in Central America."

The president did not say how he would end what he described as "a pause" in funding the rebels. The question of how to bring the contra issue back before Congress is under sharp debate in the administration. Officials hope to secure bipartisan support for attaching lethal aid the administration says the rebels desperately need to a package of humanitarian aid that House Democrats have promised.

The dilemma over a contra approach was apparent in a memo from White House chief of staff Howard H. Baker Jr. and national security adviser Lt. Gen. Colin L. Powell warning White House and National Security Council staffers against providing "assistance or encouragement of any kind to private individuals or third parties raising funds on the Freedom Fighters' behalf."

The one-page memo, disclosed by the White House yesterday, predicted that because of the House vote, White House staffers "increasingly may find themselves faced" with requests to aid private fund-raising for the contras.

"The administration will continue to work through the Congress to security the necessary aid for the Freedom Fighters," the memo said in an underlined sentence. "However, at the president's direction this is to advise no administration officials should provide" assistance.

"We believe that any such assistance or encouragement, no matter how well intentioned, would be misunderstood, misinterpreted and, therefore, counterproductive," it added.

The memo advised White House staffers not to take a position "for or against private {contra} support," not to discuss the issue and to discourage any discussions of private funding for the rebels in meetings or briefings in which administration officials participate.

White House spokesman Marlin Fitzwater said the memo does not bar White House staffers from making personal donations to the rebels. "We can't make rules regarding people's constitutional rights," he said.

The memo was made public in an apparent attempt to show Congress that the White House is taking steps to avoid a repetition of the briefings and fund-raising efforts of former NSC aide Lt. Col. Oliver L. North.

Fitzwater's briefing yesterday also touched on covert administration aid to the Afghan guerrillas fighting Soviet troops in their country.

The issue has suddenly been thrust forward with the announcement by Soviet leader Mikhail Gorbachev that his forces will begin withdrawing on May 15.

The administration has indicated that it will cease funding Afghan rebels after the Soviets withdraw their forces. But it has never spelled out precisely when U.S. funding there will end.

The New York Times reported yesterday that that the president did not approve and was unaware of the pledge to end the aid when it first was made in 1985. Fitzwater dismissed the article, saying, "One, the point of the story is wrong, and two, it's irrelevent. . . . "

The president's policy, he said, has been consistent "from Day One." He conceded, nonetheless, that the timetable for ending U.S. aid has yet to be defined.

The key to any Soviet withdrawal, he said later, is a U.S. determination that "it's irreversible."