– President Donald Trump criticized the leader of the nation’s largest union federation on Monday, escalating the feud between the administration and organized labor amid crucial negotiations for both sides over the North American Free Trade Agreement (NAFTA).

Richard Trumka, president of the AFL-CIO, had disputed on Sunday the White House’s strategy for renegotiating NAFTA and argued that Trump had “done more to hurt workers than to help” them since taking office.

Those comments elicited a sharp counterattack from Trump, who blasted Trumka as an ineffectual leader just as union members across the country prepared for Labor Day celebrations.

“Trumka, the head of the AFL-CIO, represented his union poorly on television this weekend,” Trump said in a tweet. “Some of the things he said were so again[s]t the working men and women of our country, and the success of the U.S. itself, that it is easy to see why unions are doing so poorly.”

The rift comes after the administration submitted a letter late last week formally notifying Congress that it would enter a trade agreement with Mexico, adding that Canada could be included at a later date “if it is willing.”

The letter started a 90-day process for reworking the trade deal ahead of a transition of presidential power in Mexico, as Trump seeks to fulfill a key campaign promise to revamp NAFTA ahead of the 2018 midterm elections.

The deal brokered between Mexico and the U.S. has many provisions that organized labor likes, including more robust rules for automobile ­production.

Speaking on Fox News on Friday, Trumka acknowledged that the initial deal includes some improvements but also argued that “it’s pretty hard to see how that would work without having Canada in the deal.” Talks between Canada and U.S. fell apart late last week amid ongoing disagreements over dairy rules and the trade-dispute resolution process.

Trumka also said during his Sunday interview on Fox that the economies of the U.S., Mexico and Canada are integrated. “We’re anxious to move forward with it and anxious to have all three countries involved, because NAFTA has had a devastating effect on the working people of this country for the last 25 years.”

As the head of the country’s largest federation of labor unions, Trumka is an influential voice on trade issues, and the prospects for passage of any legislation promoted by Trump are likely to diminish without his support.

It is unclear whether the president has the legal authority to withdraw from NAFTA without congressional approval.

Republican lawmakers have signaled that they are wary of any plan that does not include Canada.

Talks between the United States and Canada are set to resume Wednesday.

Other complaints

Trumka criticized Trump over the Republican tax law passed last fall, arguing it would increase the outsourcing of U.S. jobs. In addition, the union leader criticized the president for not defending an Obama-era rule mandating additional overtime pay and for not passing legislation to improve U.S. infrastructure, among other issues.

“Unfortunately, to date, the things he has done to hurt workers outpace what he has done to help workers,” Trumka said. “We’ll keep trying to find areas where we can work with him.”

Union membership nationwide has fallen markedly from the 1970s, with the percentage of U.S. workers in a union dropping from about 25 percent in the 1970s to less than 11 percent in 2017. But popular support for unions has risen steadily, to a 61 percent approval rating, a high point in more than a decade, according to Gallup polling.