DETROIT — As bargainers for the United Auto Workers and General Motors Corp. continue to haggle across a table in Detroit, the big issue in the critical contract talks comes down to this: If GM pays the union to take on the company's huge retiree healthcare obligations, can the UAW's investments return more than the rate of healthcare inflation?

GM wants to unload much of its roughly $51 billion in unfunded retiree health costs to a trust that would be administered by the union. The UAW in exchange wants promises that GM will continue building cars at union-represented plants.

It's the key obstacle of the talks, and the complexities are what's dragging them out. The two sides have yet to agree on how much GM will put into the trust, a person who had been briefed on the bargaining said yesterday. The person, who requested anonymity because the negotiations are private, said the talks likely would take several more days to complete.

Yesterday marked the fifth day of bargaining since GM's contract with the UAW was scheduled to expire, but the union has extended the pact hour by hour. Negotiators went home for the night about 9 p.m. and were to meet again today, GM spokesman Tom Wickham said.

Whatever GM and the union agree to likely will be copied when it comes to Ford Motor Co. and Chrysler LLC, and experts say other companies with large work forces also could follow.

"If this model of giving the union and its members some skin in the game to control healthcare works, then I think other companies with large future healthcare liabilities also would try to work with the unions they deal with and try to copy this model," said Glenn Melnick, a healthcare finance professor at the University of Southern California and an economist for the Rand Corp.

GM has been pushing hard for the union to take on the retiree health costs. All three automakers are interested in paying into a trust fund that the union would administer. The trusts, called Voluntary Employees Beneficiary Associations, or VEBAs, would let the companies remove the liabilities from their books and possibly raise their stock prices and credit ratings. In a recent note to investors, Morgan Stanley analyst Jonathan Steinmetz predicted that VEBAs would save the Detroit automakers $200 per vehicle.

But rapidly rising healthcare expenses might be giving the union pause. Most economists think those costs will rise 6 percent to 8 percent annually "for as far as the eye can see," said Melnick.

But market volatility, plus GM's desire to pay far less than its entire healthcare obligation — 65 percent to 70 percent, by most accounts — will make it difficult on the union, Tynan said.

"It's a tall order," he said, adding that the UAW probably will go for less-risky investments that won't bring the 7 percent or so annual return needed to cover the inflation.

Yet the United Steel Workers union has made about 40 such trusts work for several years, often at far lower rates than GM is willing to contribute.