A U.S. proposal to prevent the tobacco industry from suing foreign governments over antismoking measures threatens to imperil congressional support for a proposed 12-nation Pacific trade agreement.

With negotiations coming to a head in Atlanta, U.S. trade officials introduced language to single out the tobacco industry and allow countries to block it from using so-called investor-state dispute settlements to sue foreign governments over graphic warning labels, packaging rules and other cigarette restrictions.

The tobacco provision is receiving stiff resistance from Republican Sens. Thom Tillis and Richard Burr of North Carolina, a tobacco-growing state, and from farm-state lawmakers worried that agricultural interests in their districts could face a similar fate.

The opposition highlights the potential for lawmakers to back away from a deal that already has razor-thin support on Capitol Hill. Legislation to expedite the trade agreement through Congress passed the House this year with a 10-vote margin, but now 17 House lawmakers and a handful of senators who voted for the fast-track bill are expressing concern.

“We, along with America’s farmers and ranchers, have repeatedly stressed our deep concerns with such a slippery slope and the precedents that it would establish,” wrote House Agriculture Committee Chairman Mike Conaway (R., Texas) and 16 other lawmakers on the panel about the tobacco proposal.