In the eerie pre-dawn glow of lights from the huge new General Motors assembly plant here, the United Auto Workers and union opponents squared off at 50 yards today for one of the labor movement's critical showdowns of the decade.

At one end were 10 union supporters with armloads of literature about seniority rights. At the other end were six members of the "Team" oppbsition group with a table full of free T-shirts purchased with donations from the local business community.

As the first of the 2,300 workers who will vote tomorrow on whether to affiliate with the UAW straggled past Detroit-based corporate and union observers into the paint and body shops, both sides did a brisk - and seemingly inconclusive - business.

Forged 40 years ago by sitdown strikes and bloody encounters with company security guards into one of the nation's most powerful unions, the UAW is now locked here in a battle of T-shirts, mass mailings and a sophisticated, corporate-designed "team" approach to assembly-line work that some workers, perhaps many, see as a substitute for unions.

A UAW victory would be a shot in the arm for union labor's faltering efforts to organize the nation's fast-growing Sun Belt, which is rapidly absorbing many jobs from the heartland of union strength in the industrial Northeast and Midwest. It could also add to the union push for GM style neutrality pacts under which a company pledges not to resist union organizing of new plants - a policy that has creaked a bit under strain from the campaign here.

Considering the size and prestige of the 1.5-million-member UAW, a defeat could set back the union movement as a whole. For the UAW itself, it would mean the first non-union link in GM's nationwide chain of 19 assembly plants and a sign of weakness just as the union begins negotiating a new three-year contract with GM, Ford and Chrysler.

UAW President Douglas A. Fraser underscored the significance of tomorrow's election from the union's standpoint last Monday when he halted contract bargaining with GM in response to what he called "flagrant violations" of the neutrality pact by plant-level company officials here. This prompted both denials and stepped-up surveillance by top-level corporation officials.

Union organizers continue to complain that company officials are winking at excesses by anti-union workers, but a greater challenge to the movement appears to arise out of the plant itself: its promise of 5,000 or more high-paying jobs with work rules and benefits that, for some workers, offers an attractive dues-free alternative to the union.

The "team" concept, in particular, rallies the workers who are fighting the UAW.

The idea is to respond to workers' frustrations about the autocratic and dehumanizing assembly line by organizing them into small semi-autonomous teams, with some voice over assignments, overtime work and rules in general. The UAW has pioneered in worker-participation efforts at other GM plants, but only when the union is an equal partner in their planning and execution and has full contractual rights over wages, hours and working conditions.

Another anti-union rallying point is the jobs and pay that GM workers already have. The average GM hourly wage is more than $9, nearly half again as high as the average industrial wage in Oklahoma. Moreover, GM has recieved about 35,000 applications for the 5,000 jobs it expects to have filled by the end of the year. "If you're making $4 to $4.50 an hour, maybe you'd take a little...kicking from GM just to get even $8." said UAW organizer Carlton Horner.

The "Team" has also dwelled heavily on UAW strikes over the years and suggesting in a mailout to all Oklahoma residents, excluding workers who transferred from GM plants in other states, that the UAW may try to reinstate seniority rights of transferred workers - a charge the union angrily denied in leaflets handed out today.

Horner and other UAW organizers also complain that the business community, including The Daily Oklahoman, the dominant local newspaper, has combined with GM to foster an anti-union climate. Said Dave Bramlett, who is supporting the union after opposing other unions on earlier jobs: "You're brought up to believe unions are something foreign."

John Knowlton, a "Team" leader and former Teamster now a stockman in the chassis department, said neither GM pressure nor anti-union bias is influencing him. "The company offered me a job on my own merits, and I accepted it on the same basis," he said. "The union simply has nothing to offer us that GM hasn't already given us."

Said Olympia Costa, another "Team" supporter: "The system is working if people would only give it a chance...it takes time and people are impatient."

On the other hand, UAW supporters say the team approach has already failed. "They tell you that you make the decisions but you don't," said Linda Patton, a quality control inspector. "The main issue for our team was when and where to hold the team picnic."

A visitor to the sprawling tan plant nestled among cornfields, suburban subdivisions and oil wells is struck first by its size: 436 acres of buildings, parking lots and railroad tracks, including 70 acres (3 million square feet) under the roof of the main plant.

There are 5.6 miles of railroad track, some of it within the building, and 21 miles of assembly-line conveyor belt.

A second impression is that, despite the experimental solar conductor on the roof and the brightly colored interior, this most advanced of all auto assembly plants is far from any Star-Wars technology. Although many of the jobs require robot-like movements, they still require human hands.

The plant is producing 75 Chevrolet Citations and Pontiac Phoenixes an hour. The UAW contends this is more than comparable union-organized plants put out and means the Oklahoma workers are being pushed too hard. But "Team" supporters say the plant has scored high on quality-control tests and low in absenteeism.

The Oklahoma City plant, which opened in April, is GM's first new assembly plant in about a decade and the largest GM facility to be tackled by the UAW since the neutrality pact was negotiated three years ago. Another plant is scheduled to open in Shreveport, La., next year. "If we don't win this one," said UAW organizer Horner, "there'll be a mini-GM all through the tsun Belt - 100 percent non-union." CAPTION: Picture, Union supporters and, in background, opponents hand out leaflets about today's election at newest General Motors assembly plant, covering 436 acres in Oklahoma. AP