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.

"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.

Speakers at Wednesday's event didn't actually call for the cancellation of NAFTA, just major changes.

Lawmaker Rose DeLauro recalled having tears in her eyes the night the House of Representatives voted for NAFTA and she said the last quarter-century has proven that critics who said the deal would undermine American worker wages were right.

Trade politics in the U.S. rarely hews to clear, predictable partisan lines.

In 1994, it was mostly Republicans who helped Democratic president Bill Clinton get NAFTA ratified. Today, polls show Democratic voters are likelier to support NAFTA, but not their elected representatives, while it's the opposite on the right, where Trump voters mostly oppose NAFTA but most Republican lawmakers support it.

DeLauro rejects being painted as a protectionist or as anti-trade.

"We are for trade," said the Connecticut Democrat.

"We want to work with the administration. We want to rewrite NAFTA."

Sanders says poorly designed trade deals are one of the factors that have hurt U.S. manufacturing communities in recent decades and led to greater inequality between haves and have-nots.

"It devastates lives. And it devastates cities and towns," Sanders said.

"Trade is not the only explanation for that. But trade is an important part of the decline of the middle class America."