Donald Trump may fashion himself the world’s toughest negotiator, but federal officials are having to drag him to the bargaining table to face off with some housekeepers.

After months of stonewalling from Trump and his partners, the National Labor Relations Board ruled Thursday that Trump’s Las Vegas hotel was breaking the law by refusing to recognize its workers’ labor union. The board ordered Trump to cease and desist and start bargaining toward a contract.

Workers at the Trump International on the Vegas strip voted in favor of unionizing with the Culinary Workers Union last year, by a count of 238 to 209. Trump’s team claimed that the union unfairly influenced the results, but officials found no merit in those allegations and certified the election results.

The hotel has nonetheless refused to acknowledge the union. Now the labor board has to force it to.

Many of the workers at the hotel are Latino and Filipino immigrants working in housekeeping and food service jobs. According to the Culinary Workers Union, the Trump workers on average earn roughly $3 less per hour than other unionized workers on the strip.

In a statement Thursday, Geoconda Arguello-Kline, the union’s secretary treasurer, called on Trump to sit down at the bargaining table.

“The National Labor Relations Board has consistently told Donald Trump and his company that the union election wasn’t rigged and that he must sit down and negotiate with the Culinary Union,” Arguello-Kline said. “Mr. Trump is breaking federal law and Trump Hotel Las Vegas is operating illegally. Mr. Trump should accept the federal government’s order to negotiate and treat his workers with respect.”

Unlike most Republican presidential nominees, Trump has pitched himself as a candidate for union workers. He boasts of having “great relationships” with labor unions, and claims he can raise wages and bring high-paying manufacturing jobs back from overseas.

His dispute in Las Vegas has complicated that pitch, to say the least. As The Huffington Post first reported earlier this year, Trump and his partners there spent more than half a million dollars on high-priced consultants whose job is to dissuade workers from unionizing. Workers voted in favor of the union anyway.

The Vegas workers have accused hotel management of illegally retaliating against them during the union organizing drive and pressuring them to vote down the union. The labor board’s general counsel, who acts as a quasi-prosecutor, found merit in several of those claims.

The hotel’s refusal to bargain has led the union to take the rare step of calling for a boycott of Trump properties. At a protest in Washington last week, union member Joshua Armstead told HuffPost that he couldn’t in good conscience patronize a Trump hotel while Trump refused to bargain with his employees.

“If you want to be president of the United States, and you want to make America great again, you as an employer have an obligation to sit down with your employees,” Armstead said. “If he refuses to do that, he has no business being president of the United States.”

Editor’s note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims — 1.6 billion members of an entire religion — from entering the U.S.