Kellogg Co. and a union representing 1,400 workers failed to reach a deal after days of talks, reaching a stalemate in a monthlong strike that has disrupted operations in four factories.

The labor dispute is one of several this year in which workers are testing their strength in a tight labor market, and the Kellogg employees have drawn support from Democratic senators including Bernie Sanders and Elizabeth Warren. Earlier this week, union members at Deere & Co. rejected a deal hammered out by their own union. Nabisco workers limited production of cookies and crackers at Mondelez International Inc. for about a month before reaching a deal in September.

Kellogg shares are little changed over the course of the strike, closing Wednesday at $63.34. The company reports quarterly results on Thursday.

The cereal maker brought in contractors weeks ago to resume food production at the plants in Omaha, Nebraska; Battle Creek, Michigan; Lancaster, Pennsylvania; and Memphis, Tennessee. The locations produce cereal brands such as Rice Krispies, Raisin Bran, Froot Loops and Frosted Flakes.

Negotiations between Kellogg and the Bakery, Confectionery, Tobacco Workers and Grain Millers International Union ended “at 5:19 p.m. after the company gave us their last, best and final offer,” the labor group said Wednesday in a message viewed by Bloomberg News. “We cannot recommend this offer and will not bring it back for the membership to vote on.”

“The union continues to insist on proposals that are unsustainable and unrealistic,” Kellogg said in a statement. “They’ve proposed adding costs that would threaten the future success of our plants and cereal business.”

The company remains ready and willing to consider any realistic offers from the union, according to the statement.

The union said it “will continue this fight as long as it takes,” suggesting a a prolonged fight as workers test their strength amid widespread labor shortages and rising wages.