Boeing ( BA) says the International Association of Machinists and Aerospace Workers has seen the company's "best and final" offer ahead of a looming negotiation deadline. But when the world's largest jet-builder didn't include measures for additional job security and pension benefits in its three-year contract offer to its largest labor group, the union balked. The last time the IAM went on strike was 1995, but the union has walked out in two of the last four negotiations. If the union strikes, then Boeing will immediately cease production of all airplanes, placing thousands of other workers at risk of temporarily losing their jobs. In a message to 25,000 union members, IAM Chief Negotiator Dick Schneider termed the Boeing proposal "a continued attack on the economic health of our communities and on the jobs, incomes and living standards of the IAM workforce" and urged them to reject the offer. "Boeing put forward a job-killing, money-stealing, retiree-mugging offer," continued Schneider. "We offered to extend the contract and continue negotiations, but Boeing replied by releasing this substandard proposal to the press and public." One chasm in negotiations has been disagreement over the issue of monthly pension payments. Heading into negotiations, the union was looking for $120 a month per year worked, a major increase from the current level of $50 a month per year worked. Boeing's final offer was a 20% cumulative increase over the next three years, with $58 a month in the first year, $59 in the second, and $60 in the third. Late Sunday night, the union offered to accept $68 in the first year, which would jump to $92 by the third year of the contract. The union was also hoping for increased job security after its workforce was cut by 25% in the wake of Sept. 11. But given the new climate, in which Boeing's customers in the airline industry are asking employees for concessions and laying off staff, the jetmaker doesn't want to be handcuffed by an overly generous contract. Compromise on these issues seems unlikely. In a news conference Tuesday evening, Alan Mulally, chief executive of Boeing commercial airplanes unit, said, "This is our offer. We think it's right. We've reflected everything we can do." The IAM proposed extending its contract on a day-to-day basis so negotiations could continue, but Boeing refused to accept the suggestion, vowing to stand behind its "industry-leading" contract offer. With its contract set to expire on Sunday, the stalemate paves the way for a strike vote from the union, which, if approved, could start on Monday -- Labor Day. In midday trading, Boeing was up 37 cents, to $37.40. Despite the small gain, Boeing's stock is down 36% from its high of $50.88 on Mar. 11, as the slowdown in the aviation industry and labor woes have weighed on the stock.