By John Crawley WASHINGTON, Feb 11 (Reuters) - Mechanics at United Airlines (UAL) could vote on Tuesday to reject a contract offer based on recommendations of a presidential emergency board and, if so, would later vote to strike, their union's president said. "My sentiment tells me they will reject it," Tom Buffenbarger told reporters on Monday, referring to the 15,000 mechanics and related workers who will vote on the plan that would give them pay raises of up to 37 percent. The mechanics have not had a raise since 1994. Buffenbarger said the vote could be extremely close. But he was clear about what would happen if the company's proposal failed. "If they reject the contract, they will vote to strike," Buffenbarger said. The earliest a strike could occur would be on Feb. 20. The airline would not predict how the mechanics would vote. "We're not going to jump ahead of ourselves. We're going to see what happens tomorrow," United spokeswoman Susana Leyva said. Buffenbarger also said there would "be hell to pay" if Congress intervened in the labor organization's dispute with United by extending the deadline for a settlement or imposing contract terms. "That is not a threat. That's a promise," Buffenbarger told a news conference. He was not specific about what action the International Association of Machinists would take, but suggested it would work to rule if terms were imposed. Congress has never acted on an airline contract dispute, and Buffenbarger said he thought lawmakers would not interfere this time if asked to do so by the White House. A presidential emergency board recommended last month that the No. 2 carrier, a unit of UAL Corp. (UAL), boost pay for the mechanics to bring into line with other industry leaders. That would be roughly $35.14 per hour for the most-senior mechanics, up from $25.60. But the true value of the increase would be delayed if the financially struggling airline sought wage concessions across the board to reduce costs. United posted a staggering net loss of $2.1 billion in the fourth quarter of 2001 as it struggled with declining traffic due to the Sept. 11 attacks and the recession. Other airlines also have posted huge losses, but there have been signs of recovery recently with the gradual return of business and savings from deep cost cutting in recent months. The union is unhappy with an emergency board recommendation to defer retroactive pay for mechanics and with proposals on retirement benefits. The machinists also say the plan contains no job security provisions and they reject the proposal's blueprint for wage concessions. Buffenbarger signaled the union was willing to do its part in any giveback plan to save the airline from potential bankruptcy, but vigorously rejected the government writing that formula into its contract recommendation. The airline's contract proposal could be approved with a simple majority of "yes" votes nationwide. A strike would need two-thirds support, and Buffenbarger said it would pass easily. President George W. Bush appointed the emergency board in December to resolve the contract dispute at United. Before then, the union had voted overwhelmingly to strike. But the White House intervention delayed any walkout for 60 days.