Early voting returns on the United Auto Workers' proposed contract with General Motors Corp. showed only 51 percent support for the three-year pact, but UAW officials said yesterday that the margin is widening and that they expect approval when the voting among 350,000 union members concludes Sunday.

Rejection of the GM contract would mean a nationwide strike against the world's largest auto maker only three weeks after the UAW ended an eight-day strike against 17 GM plants on Sept. 21.

"We are cautiously optimistic," UAW Vice President Donald Ephlin said in an interview. "It is a very complicated contract . . . and the more union members come to understand it, the better chance they will support it."

As of Tuesday morning, unofficial vote counts that news organizations compiled from 31 of the 149 UAW locals showed the contract passing by a margin of 19,575 to 18,658. Union officials in Detroit said they would not release any official count until votes are finished and tallied Sunday.

But in heavy voting Tuesday and yesterday in Michigan, nine locals approved the contract by a margin of roughly 14,000 to 8,000, according to Stanley Marshall, the union's regional director in the Flint and Lansing areas. Including those votes, the contract had 56 percent support from 60,000 voters among an expected 300,000 total.

Ephlin and other officials said they believed most of the opposition comes from members dissatisfied with pay increases of 2.25 percent yearly coupled with profit sharing and cost-of-living raises. Ephlin said some workers have also complained about the "scaled" pay raises in which lower-paid GM workers will receive smaller percentage raises than higher-paid colleagues.

Ephlin, the union's chief GM negotiator, also speculated that many members were either confused by or opposed to the new $1 billion job-security "bank" and retraining program that is considered by GM and union offiicials to be the centerpiece of the contract. The job bank, slated to last at least six years, would guarantee a new job or full salary to workers who lose their jobs if the company introduces new technology, closes auto plants, or "outsources" auto parts by using foreign or nonunion sources.

"We will have job security that is unique among American industry" under the new plan, Ephlin said, but he acknowledged that many union members will not consider that provision as important as wages.

The opposition to the contract caught some UAW officials by surprise because the union's 300-member GM bargaining council had overwhelmingly approved the proposal at a meeting in St. Louis on Sept. 26. Voting by the rank and file began at union halls, churches and GM plants shortly after that vote.