Argentina's military leadership, despite having won an important defense treaty vote in the Organization of American States today, is showing increased concern that its political support both internally and internationally is slipping as the crisis in the South Atlantic continues.

Political sources said here today the military leadership that seized the islands April 2 has been painfully surprised by the continued harsh reaction by European countries and by the relatively restrained backing from Latin American allies.

Although 18 countries supported Argentina's call at the OAS in Washington today for a convocation of ministers of the 22 countries bound by the Inter-American Treaty of Reciprocal Assistance, sources said here that the Argentine government now believes it does not have enough support to pass sanctions against Britain.

At the same time, the widespread internal support that the government here enjoyed in the week after seizing the long-coveted islands showed signs of weakening today as political leaders questioned the government's negotiating position and its relations with the United States and renewed calls for major changes in economic policy.

The political situation has become paramount to the government here as it seeks to maintain its claim of sovereignty over the Falklands despite heavy pressure from the United States and a looming military challenge from Britain.

The military leadership here indicates that following the four days of tense talks with U.S. Secretary of State Alexander M. Haig Jr., Argentina has now made as many concessions as possible toward a diplomatic solution. To give up its last remaining demands for an assurance of eventual sovereignty over the islands, political sources here stress, would place the government in an almost untenable internal political position.

As Britain reacted to the new Argentine proposals for a temporary solution to the conflict, however, it appeared that Argentina would not be able to win this critical point without a rallying of international support and a firm demonstration of its willingness to stand up to the British military threat. This critical situation was described today by a leading pro-government newspaper as an alley without exit.

While the OAS met today, Argentine officials moved to rally the necessary support. A group of ambassadors here was invited to a lunch today with Argentine Foreign Minister Nicanor Costa Mendez, while the embassies of key potential allies such as Venezuela were visited by high-ranking generals.

The interior minister, Gen. Alfredo Sant Jean, met with the leaders of 13 political parties for four hours to explain Argentina's new position in the negotiations and its agreement to British participation in a temporary administration over the islands.

The government efforts came as political sources here described a worrisome picture of Argentina's political support.

Unless a military confrontation breaks out this week between Argentine forces and the British fleet now slowly making its way toward the South Atlantic, Argentina is likely to ask the OAS members to do no more than approve a resolution calling for peace and an end to the hostility, according to sources close to the military command.

While Latin American countries are nearly unanimous in supporting Argentina's claim to the islands 300 miles off its coast and its description of Britain's 149-year rule of them as a vestige of colonialism, most have shown reservations about the Argentine use of force in seizing them.

The military leadership here appers to have believed that when it launched the invasion, much of the world would accept its argument that Britain had forced a confrontation by refusing to take Argentina's claims seriously during years of fruitless negotiation.

"The problem is that this thing was done by a very small group of people, who kept it secret," said one informed source here today. "They never consulted with the people in the Foreign Ministry who could have given them an indication of what would happen. And they were not warned off by the United States because the United States did not believe there would be an invasion."

This apparent lack of foresight by the military leadership has begun to become an open issue here as political party leaders have slowly dropped their unquestioning support of the government's management of the crisis.

Political leaders in recent days have publicly expressed concern that miscalculations by the Foreign Ministry will leave Argentina internationally isolated and force it into a disastrous war with Britain.

In a carefully worded editorial this morning, the daily Clarin, which is closely identified with moderate political leaders here, said that "one thing is the rightful recuperation of a territory and another is to confront a cruel conflict in conditions of relative international isolation."

Political leaders here have also begun to question the government's negotiating position with Britain and its cooperation with the transoceanic shuttle diplomacy of Haig.

One group of political leaders here has issued a declaration saying that the United States cannot be considered as "an objective actor in the service of the peace," but only as "spokesman" for Britain's ambition to regain control over the archipelago.

Despite the meetings with Sant Jean today, leaders of a five-party opposition front also are expected to approve a public declaration calling for a dramatic reversal of the military's conservative economic policy.

The multi-party group, which includes Argentina's largest and most powerful civilian forces, dropped its campaign against the economic program after the seizure of the islands. Now, however, these political leaders and their labor allies are saying they can no longer suspend their demands as the confrontation with Britain passes through a third week.