Business and congressional liberals are gearing up for a major battle over health insurance for former employees. Under a law known by its acronym, COBRA--taken from an otherwise obscure budget bill--companies must allow departing workers to buy benefits at group rates for 18 months. Employers say that because the former workers who take the coverage are often older or sicker than average, COBRA drives up premium costs. That's why they're worried about efforts to expand the law. Representatives Nancy Pelosi (D-Calif.) and Patricia Schroeder (D-Colo.) are pushing for changes that would allow spouses over age 50 who lose benefits because of death or separation to maintain their insurance coverage indefinitely.