In all, 5,000 Starbucks employees will go on strike in more than 25 states on Tuesday, spanning from Maryland to Montana to California, Workers United said.
Workers in Columbus, Ohio, Cheyenne, Wyoming, Buffalo, New York, and a host of other locations are set to join the strikes, the union said.
The work stoppages on Christmas Eve mark the final wave of a five-day strike meant to disrupt Starbucks during one of the busiest times of the year for the coffeehouse giant.
"These strikes are an initial show of strength, and we're just getting started," Lauren Hollingsworth, a Starbucks barista in Ashland, Oregon, told ABC News in a statement.
Starbucks Workers United and Starbucks announced earlier this year that they would work on a "foundational framework" to reach a collective bargaining agreement for stores, something the union says has not come to fruition.
"We were ready to bring the foundational framework home this year, but Starbucks wasn't," Lynne Fox, President of Workers United, told ABC News in a statement.
The strike began on Friday and has escalated each day since. On Monday, about 60 stores were forced to close as result of work stoppages, the union said.
In response to ABC News' request for comment, Starbucks Spokesperson Jay Go Guasch said the strikes had impacted a fraction of its U.S. stores.
"Only around 170 Starbucks stores did not open as planned. With over 10,000 company operated stores, 98% of our stores and over 200,000 green apron partners continuing to operate and serve customers during the holidays," Go-Guasch said.
Sara Kelly, Starbucks' executive vice president and chief partner officer, downplayed the impact of the strikes in a public letter to employees late Monday.
"The overwhelming majority of Starbucks stores across the country have opened as planned and are busy with customers enjoying the holidays," Kelly said, noting that the company operates 10,000 stores and employs 200,000 people nationwide.
Anticipating the expansion of the strike on Tuesday, Kelly said work stoppages in hundreds of stores would cause "very limited impact to our overall operations."
"The union chose to walk away from bargaining last week," Kelly said. "We are ready to continue negotiations when the union comes back to the bargaining table."
The union and the company remain far apart on the key issue of potential wage increases, according to statements from both sides about the other's proposal.
Workers United told ABC News in a statement that Starbucks had proposed no immediate wage increases for most baristas and a guarantee of only 1.5% wage increases in future years.
Meanwhile, Starbucks said in a statement that the union had proposed an immediate increase in the minimum wage of hourly partners by 64%, as well as an overall 77% raise over the duration of a three-year contract. "This is not sustainable," a Starbucks spokesperson told ABC News.
Starbucks United contests those figures as a disingenuous characterization of its proposal, the union told ABC News.
Baristas have unionized more than 100 Starbucks stores this year, expanding a union campaign that has spread to hundreds of stores across 45 states since an initial victory three years ago at a location in Buffalo, New York, the union said.
The union has filed hundreds of charges with the National Labor Relations Board alleging illegal anti-union activities carried out by Starbucks, including alleged bad-faith negotiations over a potential union contract setting terms at the unionized locations.
Starbucks has denied wrongdoing and faulted the union for breaking off negotiations. The company offers better pay and benefits than its competitors, Starbucks said.