Donald Trump has pledged to rewrite NAFTA so that the trade agreement does more to protect American workers from unfair competition. Specifically, the president has evinced a desire to prevent “American” jobs from being sent to Mexico, where workers are (typically) willing to accept lower wages.

Canadian prime minister Justin Trudeau agrees that NAFTA does not do enough to establish common labor standards for the North American continent. But he doesn’t think that Trump needs to look quite so far south to find a place that’s undercutting American workers: In 28 U.S. states, so-called “right-to-work” laws inhibit the ability of workers to unionize, thereby holding down wages, and encouraging companies in America’s other 22 states to ship jobs across our nation’s (internal) borders.

These laws undermine organized labor by allowing workers who join a unionized workplace to enjoy the benefits of a collective bargaining agreement without paying dues to the union that negotiated it. This has the effect of encouraging other workers to skirt their dues, which can then drain a union of the funds it needs to survive.

Trudeau has called for a revised NAFTA to prohibit such laws, so as to protect Canadians from losing jobs to cheap American labor.

On Wednesday, Democratic senators Elizabeth Warren, Sherrod Brown, and Kirsten Gillibrand announced their agreement — and introduced legislation to ban “right-to-work” laws throughout the United States.

“If we want to protect workers and expect a level playing field in international trade deals, we need to start at home,” Warren told reporters Wednesday, “and that means banning states from imposing restrictions that prevent workers from joining together to fight for their future.”

Unfortunately, those 28 right-to-work states have 56 Senate votes. So, we’re probably gonna keep depressing North America’s labor standards for the foreseeable future.