Jorge Triacca, the burly former plastics worker who heads Argentina's largest and most moderate labor confederation, planned to be leading several million workers in a historic work stoppage against the troubled military government Wednesday.

That was before the military seized control over the Falkland Islands and plunged Argentina into a militant confrontation with Britain. Now Triacca says he will convoke not a paralyzing strike but a press conference, and he will substitute his harsh criticisms of the government with an offer to turn the offices of his national union into centers for military recruitment.

"We want to show that we are willing to fight here as our sons and brothers are fighting in the south," said Triacca, whose union only last Thursday had agreed to join the swelling domestic opposition to the administration of Gen. Leopoldo Galtieri. "This is something that surpasses all factional troubles; this is a historic claim of all the Argentine people."

But the joy and pride described by political leaders and newspapers is not so clearly evident along the streets of Buenos Aires. No popular demonstrations have followed the first gathering organized by union leaders last Friday, and while officials have described lines of eager volunteers forming at military recruitment offices, the central Army office was filled at noontime yesterday only with the usual crowd of youths seeking deferments.

Even so, Triacca's sudden reversal from militant dissidence to patriotic support for Argentina's armed forces is being repeated this week by labor and political leaders all through this emotionally charged country.

While suffering a string of diplomatic reversals since ordering the armed occupation of the Falklands last Friday, Galtieri's government has managed to rally crucial support from labor and political leaders who only days before were demanding an end to military rule and battling government troops in the streets.

This political unity, unprecedented in the six years of military rule here, has come to be as potentially significant within Argentina this week as the standoff with Britain.

High-ranking military and political leaders met for three hours in the presidential palace last night. Today, political sources said the military, anxious to assure continued Argentine solidarity as a British naval fleet sets out for the Falklands, could soon make major political concessions or even agree to form a new government with participation of political parties.

"Argentina is a country where 20 years of history can happen in one day," said a high official of Triacca's National Commission of Labor. "Because we have no stable political system, Argentines react emotionally to events like this and anything can happen."

As the National Commission of Labor was calling off its strike today, the leaders of Argentina's other national union federation and most dogged government opponent, the General Confederation of Labor, was meeting to draw up a communique expected to renew calls for unity with the military in the crisis.

The federation's leaders helped organize a demonstration last Friday to celebrate the taking of the islands, even though three days earlier a huge antigovernment demonstration they organized at the same site resulted in some 1,500 arrests and considerable violence.

The front of civilian political parties that formed in opposition to six years of military rule also planned to meet to call off a major protest planned for later this month, according to political leaders.

Labor and party leaders, whose protests had seemed destined to create a major political crisis for the government before the invasion began, are careful to make the point, as Radical Party leader Carlos Contin put it, that "we are expressing solidarity with ourselves, the people, and not the military junta."

Contin and other leaders said their loudly expressed unity with the government over the taking of the Falklands is based on nearly unanimous public support for the forceful invasion of the windswept territory that Argentina had sought peacefully to reclaim from Britain for 149 years.

"This is a question of national sovereignty, so the people stand united," said Contin. Previous Radical Party governments frequently used the Falklands as a rallying cry.

But in casual conversation, many Argentines here express only a cynical apathy to the Falklands takeover, or focus on concerns that the advancing British warships might plunge Argentina into a bloody war.

"I support the government, but the Malvinas mean nothing to the common man," said one worker outside the General Labor Confederation headquarters. "The Malvinas are not going to raise my salary or lower the taxes or bring down the inflation that is wiping us out."

Civilian leaders deny such sentiments are widespread, but at the same time admit that their support for the government's initiative may be based as much in Argentine history and tradition as in popular fervor.

In Argentina, isolated both geographically and politically from most of the Western world, a strong tradition of nationalism has characterized political parties for generations. It is strong in the movement of former president Juan Peron, which still controls most of the labor movement.

In addition, the country's claim over the Falklands has become such a symbol of nationalism over the years that political leaders say it is considered impossible politically to oppose any initiative to incorporate the islands.

"We are very pressured," said one leading labor official. "We have to drop whatever measure or confrontation with the government when it picks up an issue like this. There is no other political course."

Political leaders said they now expect the crisis and their willingness to cooperate with the government to gain them more than the strikes and demonstrations they have been planning all year. "After coming to us and asking for our help," the labor leader said, "they will have to continue working with us when the crisis is over, or else there will be a rebellion."

"This unity among all the sectors is going to prolong and maintain itself even after the solution is reached," said Contin.