Secretary of State George P. Shultz and CIA Director William J. Casey offered Congress a new justification for covert U.S. activity in Nicaragua yesterday, stating that its purpose is to pressure the leftist Sandinista government into stopping its efforts to export revolution.

Sources familiar with the closed-door presentation to members of the Senate Intelligence Committee said the new "finding" is a significant shift in emphasis from previous administration explanations that covert U.S. aid to anti-Sandinista rebels was aimed at stopping the flow of arms from the Sandinistas to leftist guerrillas in El Salvador.

Instead, the sources said, the action will now be justified as necessary as long as Nicaragua continues to help guerrillas elsewhere in Central America.

The new justification, first reported by The Washington Post in July, is consistent with recent Reagan administration demands that the Sandinistas change their overall behavior in Central America. The administration has vehemently denied that it supports the avowed goal of the anti-Sandinista rebels to overthrow the Nicaraguan government.

In a statement after yesterday's three-hour meeting, Intelligence Committee Chairman Barry Goldwater (R-Ariz.) said he expects the committee to decide whether to approve the new finding by the end of the week. Approval would put before the Senate the question of continuing the estimated $80 million program of aid to anti-Sandinista rebel forces.

The committee voted last May to continue funds for covert action in Nicaragua after Oct. 1 only if President Reagan provided "a redefined position on Central America," Goldwater said at the time. "We want him to tell us in plain language just what it is he wants to do relative to Nicaragua and the other countries."

The sources said yesterday's meeting was "not heated" despite the presence of several prominent administration critics and some "very tough questioning." They said committee members "seemed inclined to support the proposal."

On the House side, former CIA director William E. Colby urged the Intelligence Committee to reject proposed new curbs on the agency's covert actions abroad. If Congress has problems keeping track of what the agency is doing, he said, "you have to take a two-by-four to the head of the mule . . . , but I don't think this is the two-by-four."

Colby was the opening witness at three days of committee hearings on legislation sponsored by Rep. Wyche Fowler Jr. (D-Ga.) that would require the administration to give prior notification to the House and Senate Intelligence committees of any covert action and would give them a veto power over it. Existing law requires only that Congress be "fully and currently informed" of "significant" activity, and offers no remedy other than a fund cutoff after the fact.

Covert action also would have to be preceded by a written presidential finding to the committees that the action is essential to U.S. security, consistent with public U.S. foreign policy, and needed despite its risks because extraordinary circumstances mean that overt or less sensitive tactics cannot accomplish the goal. Wartime operations would be exempt from the restrictions.

These provisions, Colby said, "will ensure that no clandestine activity. . . will ever take place." Colby, who was CIA director from 1973 to 1976, said he had no quarrel with the imposition of standards upon the beginning of a covert action and said Fowler's proposals were "very good" standards.

"My only question is whether you want to absolutely set them into legal concrete," he said.