The United Auto Workers union overwhelmingly won a sharply contested election to represent blue-collar workers at a huge new General Motors assembly plant here today in a big boost for organized labor in the nation's Sun Belt.

The 1,479-to-658 vote keeps the UAW stamp on all of GM's 19 assembly plants and strengthens the hand of the powerful 1.5-million-member union as it begins bargaining for a new three-year contract with GM, Ford and Chrysler.

The margin was bigger than either the UAW or its opponents had expected. It came against a backdrop of strong antiunion sentiment here and a record for unions as a whole of winning less than 50 percent of recent representation elections.

GM remained officially neutral in the campaign under a hands-off pact negotiated with the union three years ago. On Monday, UAW President Douglas A. Fraser broke off contract bargaining with GM until after the election, charging that the company was permitting "flagrant violations" of the pact at the Oklahoma City plant.

Friction continued but there were no more top-level complaints. The inplant opposition was organized bya group of workers who argued that the union was unnecessary in light of worker-participation innovations introduced unilaterally by GM when it opened the plant in April.

"We didn't campaign against General Motors; we campaigned against the Daily Oklahoman [newspaper] and the Oklahoma City Chamber of Commerce," Carlton Horner, the union's chief organizer for the plant, told several hundred jubilant UAW uaw supporters in announcing the vote at a victory party. Then he added: "Oklahoma City, you'll learn to love us."

In a statement from Detroit, Fraser hailed the victory as a boost for southern organizing efforts but said it "does not resolve...the questions we have about GM's compliance with its neutrality commitment."

GM's contract with the UAW will be implemented at the plant, which now has 2,300 hourly workers and will expand to 5,000 this fall, as soon as the National Labor Relations Board certifies the election results and the company signs a memorandum of understanding with the union.

Workers at the plant currently earn the $9 average hourly wage that the union negotiated at other plants, but will pick up such extras as supplementary unemployment benefits in the event of a layoff. As a newly organized plant, it is exempted from shutdown if the UAW strikes GM this fall when its contract with the "Big Three" auto companies expires.

The plant, which produces the Chevrolet Citation and Pontiac Phoenix cars, was only the second Sun Belt facility to vote in the UAW since the neutrality pact was negotiated in 1976. The victory is expected to give impetus to the union in organizing plants in the Deep South, including a steering-gear facility in Decatur, Ga., and another new assembly plant in Shreveport, La.