Starbucks baristas walked off the job in major cities in Massachusetts, Texas and Oregon on Monday, expanding the dayslong holiday strike to 12 cities nationwide, according to the union Starbucks Workers United.

Workers went out on strike in Boston, Dallas-Fort Worth, Texas, and Portland, Oregon, joining baristas in cities spanning from Los Angeles to Philadelphia.

Striking baristas brought business to a halt in almost 50 stores nationwide on Sunday across multiple cities, Workers United said.

“The holiday season should be magical at Starbucks, but for too many of us, there’s a darker side to the peppermint mochas and gingerbread lattes,”Arloa Fluhr, a longtime Starbucks employee in Illinois, said in a statement to ABC News.

Fluhr, a mother of three, struggles to support her family with the wages received from Starbucks, she said. “That’s why we’re steadfast in our demands for Starbucks to invest in baristas like me,” she added.

Workers United, a union representing 525 Starbucks stores in the U.S., said baristas nationwide launched a strike on Friday. The escalation on Monday is the latest expansion of a strike that has grown each day since it began, the union said.

The holiday season is one of the busiest periods of the year for the coffeehouse giant, the union added.

In February, Starbucks Workers United and Starbucks announced 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. “Union baristas know their value, and they’re not going to accept a proposal that doesn’t treat them as true partners.”

Starbucks did not immediately respond to ABC News’ request for comment.

In response to ABC News’ previous request for comment, Starbucks spokesperson Phil Gee said the company has not experienced a significant impact from the strike.

“We are aware of disruption at a small handful of stores, but the overwhelming majority of our U.S. stores remain open and serving customers as normal,” Gee said on Dec. 20.

Starbucks said it remains willing to resume negotiations with the union. “Workers United delegates prematurely ended our bargaining session this week. It is disappointing they didn’t return to the table given the progress we’ve made to date,” the company said. “We are ready to continue negotiations to reach agreements.”

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.

Some local elected officials joined workers on the picket lines on Sunday, including Democratic Pittsburgh Mayor Ed Gainey and Democratic New York City Comptroller Brad Lander, the union said.

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.

“We are focused on enhancing the partner (employee) experience, with over $3 billion invested in the last three years. Starbucks offers a competitive average pay of over $18 per hour, and best-in-class benefits,” Starbucks said in a statement to ABC News. “No other retailer offers this kind of comprehensive pay and benefits package.”

ABC News’ Leah Sarnoff contributed to this report.