By Allison Lampert

(Reuters) - A tentative labor contract agreed between Boeing and one of its biggest unions has angered many workers who were hoping for higher wage hikes and better pensions, an official who negotiated the deal said on Monday.

The International Association of Machinists and Aerospace Workers (IAM), which represents more than 32,000 workers in the U.S. Pacific Northwest, announced the deal along with Boeing on Sunday, sending the aerospace giant's shares higher on Monday.

It was not clear if IAM members would vote to accept the deal, or choose to strike when they hold a vote on Thursday, said Jon Holden, president of the IAM's district 751 and lead negotiator on the Boeing contract.

"They're angry," Holden told Reuters, adding that he believed it was the best deal the union could get in bargaining.

"The power is within our membership on the floor," he said when asked if he thought the deal would be ratified.

The proposed four-year contract included a general wage increase of 25% and a commitment by Boeing to build its next commercial airplane in the Seattle area, providing the plane program was launched within the four years of the contract.

Holden said many members wanted to hold out for a 40% pay rise over the contract period and a reinstatement of the defined-benefit pension plan they reluctantly gave up during a round of negotiations a decade ago.

"It's hard to come off of 10 years when you lost so many things that were critical," Holden said.

If union workers vote down the deal and decide to strike, it would be a blow to new Boeing CEO Kelly Ortberg, who took up his role last month with a mandate to improve safety and ramp up production of Boeing's best-selling 737 MAX passenger jet.

Boeing is wrestling with a quality crisis and faces scrutiny from regulators and customers, after a door plug on a near-new MAX blew off an Alaska Air jetliner while in mid-air in January.

(Reporting by Allison Lampert; Writing by Joe Brock; Editing by Jamie Freed)