Galtieri's power: only as firm as his grip on economy?

Argentina's new military leader is proving a tough and wily strategist as well as a potentially warm ally of the United States. Gen. Leopoldo Fortunato Galtieri's assumption of power in Argentina late last week was a well-orchestrated maneuver.

In the two weeks before he was named president Dec. 11, he engineered the retirement of five rival generals and replaced them with officers widely regarded as supportive of himself and of a tough line on human-rights issues.

He also made overtures to the influential Argentine Navy, which has long been at odds with the Army. In tapping a top admiral for the important Interior Ministry post, which handles internal security, he gave the Navy a key role in his government. Significantly, Adm. Carlos Lacoste also is serving as interim president until General Galtieri formally takes the presidency next week.

Meanwhile, General Galtieri's seizure of power appears to have the blessing of Washington. He made two little-known trips to the US in recent months, including one in early November when, as Army chief of staff, he lunched with top Reagan administration officials.

Washington and Buenos Aires are on a much more friendly tack than during the Carter administration, when the human-rights record of the Argentine military was a major deterrent to good relations. About 6,000 Argentinians are reported to have disappeared during that time. Families of those who disappeared are still trying to track down their loved ones, but no disappearances have been reported in the past year or so.

In exchange for US friendship, General Galtieri has already begun to take a role in Central America, agreeing to train Salvadoran combat forces. Sources in Buenos Aires also indicate that he promised the US an Argentine contingent in the peacekeeping force in the Sinai, called for by the Camp David accords.

In addition, it is understood that General Galtieri assured the Reagan administration that the growing trade between Argentina and the Soviet Union does not intimate a political relationship - something the Reagan administration worries about.

The problems General Galtieri faces as Argentina's new strong man are many. Economic ones are foremost. In the weeks before he ousted Gen. Roberto Eduardo Viola as president, General Galtieri repeatedly complained about mismanagement of Argentina's stagnant economy. These complaints helped discredit the Viola government.

But the problems are now his. General Galtieri will be watched carefully in the months ahead to see how he comes to grips with: inflation, which runs more than 100 percent a year; this year's 5 percent drop in gross domestic product; the flight of capital and brains. Carping about his performance can be anticipated if solutions are not quickly forthcoming.

This being Christmas time, and with the holiday period extending into January , he does have a brief honeymoon period. Moreover, General Galtieri has covered his flanks with the appointments of supportive officers.

''Retiring'' rival generals Antonio Domingo Bussi and Jose Rogelio Villarreal , before ousting General Viola, assured him of an easier road to the presidency. And his subsequent naming of friendly generals to key posts gives Galtieri breathing room in the days ahead. Among the new appointments are Horacio Jose Varela Ortiz as Army chief of staff and Cristino Nicolaides as head of the important First Army.