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

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

Those comments elicited a 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 starts 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 have earned the support of organized labor, including more robust rules for automobile production.

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

Trumka also criticized Trump over the Republican tax law passed last fall, arguing it would increase the outsourcing of American jobs.