Retirement healthcare for as many as three-quarters of a million Americans will be placed at high risk if conditions proposed as part of auto rescue loans are enforced by the incoming Congress and Obama administration, labor experts say. At issue is a condition of the loans that calls for General Motors Corp. and Chrysler to use company stock or the equivalent to pay half, or $10.5 billion, of the cash owed to a union retiree health care trust. The last company to use that strategy was Enron, and workers there lost the lion's share of their retirement savings when the company's once fast-gaining stock became worthless.