Bill de Blasio is talking trade.

"These trade agreements, starting with NAFTA, have decimated the lives, decimated the quality of living of so many American workers," de Blasio said Thursday, during a campaign rally alongside union workers.

It's not the first time de Blasio railed against the North American Free Trade Agreement. Back in 2016, during that presidential race he also opposed the agreement. Now, with the White House in his sights, de Blasio is challenging every Democrat seeking the nomination to oppose President Donald Trump's re-negotiated version of the trade agreement.

"We learned that the old NAFTA didn't work. We sure as hell know that new NAFTA is not going to work, right?" he said.

De Blasio did not lay out any specific proposals that would help many of the workers who stood beside him during this rally, many of them belonging to key manufacturing sector jobs, that could technically be affected by trade.

Just a few blocks east, former vice president Joe Biden was in Manhattan, laying out major foreign policy proposals and criticizing President Donald Trump.

"American foreign policy has to be purposeful and inspiring, based on clear goals driven by sound strategies, not by Twitter tantrum," Biden said.

De Blasio, flanked by union workers, took a swipe at Biden, who continues to be the front-runner in the polls.

"There are some Democrats who supported NAFTA. Joe Biden is one of them. I respect him, but he has to explain to people why he did that and what he's learned since then," de Blasio said.

The Hotel Trades Council, the only union to endorse the mayor, came out in support. Leaders from the United Automobile Workers, the Retail, Wholesale and Department Store Union, and the Communication Workers of America were in attendance, but not ready to endorse de Blasio's presidential bid.

"We love Mayor de Blasio, but as you know, we endorsed Bernie Sanders last time, we love Elizabeth Warren, we love a lot of candidates, so we'll see what happens," said Bob Master, political director for the Communication Workers of America.

De Blasio is back on the campaign trail this weekend. He'll be stumping for votes in Iowa ahead of a key fundraising deadline next week.