Union leaders expect a close vote when 33,000 Boeing hourly workers weigh a sweetened contract offer that could end an almost six-week strike, the union's chief said Tuesday.

"I think it's going to be a tight vote," Jon Holden, president of the Seattle-based machinist union told CNBC on the eve of Wednesday's vote.

"There's a lot of emotion tied here," Holden said. "We've been able to negotiate the best that we could and we're hoping our members will consider it."

The latest Boeing offer includes a 35-percent pay raise over four years and a one-time signing bonus of $7,000. However, the deal does not restore the pension, a major sticking point for older workers.

The union, the International Association of Machinists and Aerospace Workers District 751, walked off the job on September 13 after overwhelmingly rejecting an initial offer that included a 25-percent wage increase. Workers had been seeking a 40-percent wage hike.

The strike has halted work at two Seattle-area assembly plants and production of its 737 MAX and 777 planes. Last week, Boeing supplier Spirit AeroSystems said it would furlough staff as a result of the stoppage.

Mike Corsetti, a quality inspector in Everett who has worked at Boeing for 13 years, plans to vote "no" because of Boeing's refusal to negotiate on the pension and concerns that the deal does not do enough to guarantee that health care costs will be limited.

"If they're not going to budge on the other things, then they need to exceed what we're asking for on pay," he told AFP.

But Paul Janousek, an electrician at Everett who has also worked for Boeing for 13 years, said he was leaning towards voting "yes."

"We're really close. I don't think we can make much more out of them," said Janousek, who expressed concern about the impact of the strike on aerospace suppliers.

The vote will take place throughout Wednesday, with the outcome expected to be announced that night.

Holden told CNBC he is prepared to return to the bargaining table if members reject the contract again.

"We'll continue to push for the things our members say are important and we'll continue to strike," he said. "That's our only option and our members will make that choice."