The rejected offer included an immediate pay raise of as much as 37 percent, but required mechanics to give back some of those wages in the near future to aid the struggling airline.
The contract was voted down by 68 percent of the 13,000 union members, represented by the International Association of Machinists and Aerospace Workers District Lodge 141-M. In a separate vote, 86 percent of the membership authorized a strike, the union said.
A strike could begin on Feb. 20. UAL Corp., the parent of United Airlines, has been negotiating with the union for nearly two years.
"Their proposals fell short of what we felt we could achieve in direct negotiations," said Scott Ford, District 141-M president and lead negotiator. "The results of today's vote shows our members agree."
After the vote, Tom Buffenbarger, international president of the Machinists Union, urged an immediate resumption of negotiations with United.
"We will travel anywhere, and meet around the clock if necessary for the next six days to reach an agreement," Buffenbarger said. "My advise to Congress and the president remains the same: stay out of this and we will get it done."
Three weeks ago, UAL accepted recommendations from a specially appointed presidential board that the airline offer mechanics raises, which the airline had said it could not afford. UAL had a 2001 net loss of $2.1 billion.
Under the offer, a top United mechanic paid $25.60 an hour would receive a raise to $35.14 immediately and to $37.54 by mid-2004. American Airlines, the nation's largest carrier, pays its mechanics $34 an hour.
However, several union lodges recommended a "no" vote because of looming wage givebacks and language in the contract that would allow the airline to delay retroactive pay increases dating to July 2000 until April 2003.
Jack Creighton, United's chief executive officer, said both parties have agreed to re-enter negotiations.
"We regret that members of the IAM have rejected the PEB's recommendations," Creighton said. "We will continue to work with all our unions to aggressively implement a strategic recovery plan that meets the needs of our passengers, preserves jobs and puts the company on the road to financial stability."
He added that the company anticipated another contract vote by the union in mid-March, "without interruption of operations."
United mechanics last walked off the job in 1979. That walkout lasted 59 days.
UAL shares lost 30 cents, or 2.36 percent, to close Tuesday at $12.39 on volume of almost 1.5 million shares traded on the New York Stock Exchange.