The International Association of Machinists is applauding a new deal covering 28,000 workers at United Airlines, but says it cannot seem to make any headway on two US Airways contracts.

The United deal runs through 2016 and covers fleet service workers, agents, and others. It provides wage boosts between 19% and 56% over the life of the contract, including immediate increases between 7% and 29%, the IAM said. With about 65% of eligible members voting, the contract won approval in excess of 70%, the union said.

“United took the approach of trying to reach agreement with the IAM so they can achieve the benefit of having an integrated work force,” said IAM spokesman Joe Tiberi. “US Airways made a mess of the last merger and is starting this potential merger with American headed in the wrong direction, again.”

Among US Airways unions, the IAM is not only the largest but also the only one that has not voiced support for the pending merger with American. Some experts credit public displays of support, including union support, with putting pressure on the Justice Department to drop its lawsuit opposing the merger. A hearing in U.S. District Court in Washington is scheduled to begin Nov. 25.

Workers at United rejected an earlier contract agreement, which followed a 2011 representation election when the IAM defeated the Teamsters following the merger between United and Continental . IAM had represented fleet service workers at United while IBT had represented the group at Continental. “After our members rejected the first agreement, we went back to the table and got an agreement they accepted,” Tiberi said. “United wanted to get a deal, and the way negotiations were handled set an example.”

In the case of US Airways, Tiberi said the union and company negotiators met this month in Dallas to discuss the mechanics contract. But the talks were hampered by the lack of a federal mediator, who did not appear due to the government shutdown. Tiberi said the company had indicated it would offer a contract proposal at the meeting, , but then did not. “They showed up without a proposal,” he said.

Talks on the contract covering about 3,500 mechanics and related workers are scheduled to resume next week, with a mediator present, while talks on the contract for about 5,900 fleet service workers will resume in November.

“Negotiations with the IAM are currently under the auspices of the National Mediation Board,” said US Airways spokeswoman Michelle Mohr. “We look forward to the next scheduled sessions and welcome the opportunity to continue that dialogue”

Contract talks at US Airways have been stalled since contracts for the two work groups became amendable 22 months ago on Jan. 2, 2012. Talks have been delayed by the airline’s merger efforts and by a failed effort by the Teamsters to organize the mechanics. That effort began late in 2012 and ended in August, when the Teamsters lost a representation election.

At United, the IAM also represents about 1,500 fleet technical instructors, maintenance instructors, food service and security officers, who remain in negotiations.

In mid-afternoon trading on Tuesday, US Airways shares were up 21 cents to $22.22 after the carrier said it has sought mediation in the DOJ’s merger case. Shares are up 65% for the year. United shares were up $1.53 to $37.65, perhaps reflecting investor approval of the contract approval. Shares are up 44% this year.