Bernie Sanders and other members of the anti-NAFTA left attend a news conference on Capitol Hill in Washington, D.C, on Dec. 13, 2017.

WASHINGTON — Donald Trump is being encouraged to stay strong in NAFTA negotiations by people who aren't normally his allies. Sen. Bernie Sanders and other members of the anti-NAFTA left held a news conference Wednesday on Capitol Hill where they demanded that the president keep his promise to drastically overhaul the agreement. They held their event as negotiators from the three countries met in Washington. "We are here today to send a very loud and clear message to Donald Trump: for once in your life keep your promises," said Sanders, a Vermont senator and former presidential contender.

Bloomberg via Getty Images Senator Bernie Sanders speaks during a House-Senate conference meeting on the Republican led tax reform bill at the U.S. Capitol in Washington, D.C. on Wednesday.

"We need to fundamentally rewrite NAFTA." Sanders expressed support for hardball U.S. negotiating positions like increased Buy American protections, and he wants to go even farther than Trump and not just water down the investor-state dispute mechanism in Chapter 11 but end it entirely. He also wants Trump to go farther on labour standards and to end incentives to outsource jobs. Those disagreements on specifics illustrate that while Trump and the left share similar rhetoric about trade, some of their specific proposals veer in different directions, raising doubt about how many left-wingers Trump could hope to win over in a ratification vote for a trade deal.

Jonathan Ernst / Reuters U.S. President Donald Trump welcomes Canada's Prime Minister Justin Trudeau on the South Lawn before their meeting about the NAFTA trade agreement at the White House in Washington on Oct. 11, 2017.