(NewsNation) — Union workers represented by United Auto Workers at Mack Trucks could strike as early as Monday with the current agreement expiring Sunday night.

The current agreement was signed after a two-week strike in 2019. The walkout happened around the same time as a UAW strike against GM which lasted 40 days.

The ongoing auto workers’ strike has focused largely on the three largest Detroit automakers — General Motors, Ford and Stellantis. But the expanding situation involving 3,900 union members at Volvo Group North America facilities in three states has drawn little attention.

“Our members at Mack voted by 98% to authorize a strike, so unless the company gets serious, they’re about to learn firsthand that our union’s back in the fight, and we’re not backing down to anybody,” UAW President Shawn Fain said referring to a strike authorization vote earlier in September.

Mack is part of the Greensboro, North Carolina-based Volvo Group. Most of the workers who would be affected by a strike are at Mack’s Lehigh Valley Operations in Pennsylvania.

“So far, the negotiations have been painfully slow,” union leaders wrote in a letter Thursday to members and posted on the Local 677 website. “Aside from their counters on wage demands, the company has rejected almost every demand of consequence that we proposed.”