A Starbucks cafe in Mesa, Arizona, has voted to unionize, marking a significant setback for the coffee chain.

• A union was formed at a Starbucks location in Mesa, Arizona, as workers voted in favor of it.
• The third company-owned Starbucks location has voted in favor of unionizing, marking the first time this has happened outside of Buffalo, New York.
• The union’s next challenge is negotiating a contract with Starbucks.

In Mesa, Arizona, a coffee chain faced a blow as workers at one of its locations voted to form a union, joining a growing movement of company-owned cafes organizing against the chain.

The cafe on Power Road and Baseline Road in Mesa, with 25 votes to three, was unionized by its employees under Workers United, a branch of the Service Employees International Union.

The third company-owned Starbucks location has voted in favor of unionizing, marking the first time this has happened outside of Buffalo, New York.

Over the past six months, more than 100 Starbucks locations have filed for union elections, increasing their count by double in the last month alone after successful elections in Buffalo. These cafes represent a small portion of Starbucks' U.S. footprint, which includes nearly 9,000 company-owned restaurants. However, this trend suggests that the restaurant industry's historically low unionization rate may rise.

A union election has been filed for at a second store in Mesa, Arizona. Ballots from the National Labor Relations Board were distributed on Friday afternoon and must be returned by March 18.

The union must now negotiate a contract with Starbucks after the National Labor Relations Board's regional director certifies the ballots, which could take up to a week. However, labor laws do not mandate that the employer and union reach a collective bargaining agreement. Additionally, workers who lose faith in the union can petition to decertify after a year, adding pressure to the negotiations.

Starbucks' North American head, Rossann Williams, wrote a letter to all U.S. baristas, stating that the company would engage in "good faith" bargaining.

Liz Alanna, a shift supervisor at the Mesa store, stated, "We are eager to commence negotiations with Starbucks, but we are also aware that Starbucks is fiercely opposing us." She added, "We urge Starbucks to cease their hostility against the labor movement and collaborate with us, not against us."

Starbucks did not immediately respond to a request for comment from CNBC.

Andy Barish, a Jefferies analyst, stated in a Thursday note to clients that unionizing does not seem to pose a significant financial risk to Starbucks in terms of substantial hourly wage hikes or benefit claims. Nevertheless, the company could face harm to its reputation if it mishandles the process.

Starbucks Workers United has accused the company of union busting. However, Starbucks has denied these allegations. The company has sent executives to Buffalo and Mesa stores to oppose the union effort.

Barish wrote that it is difficult to envision this issue escalating into a negative PR storm for SBUX, but it does pose a near-term "headline risk" for the stock, which has been underperforming recently.

Over the past year, Starbucks' stock price has dropped by 11%, resulting in a decrease in its market value to $106 billion.

The vote count for three more Buffalo-area Starbucks locations and Mesa's Starbucks location has been delayed due to Starbucks' appeals to the National Labor Relations Board. The results were originally expected to be announced on Feb. 16.

The Elmwood location in Buffalo, New York, was the site of the first union victory for Starbucks employees working in a company-owned store.