After laying out the Trump administration's most aggressive NAFTA demands to date, chief U.S. negotiator John Melle was asked on Sunday how things are progressing. "Fabulous," he said, smiling and shrugging before entering a negotiating room once more.

The fourth round of negotiations is nearing an end amid rising tensions after the U.S. presented proposals that could be politically unfeasible for Canada and Mexico. U.S. industry and Congress, meanwhile, are mounting a more vocal defense for preserving regional trade ties as they sense the discussions could be in trouble.

U.S. negotiators in recent days put forth a string of bold proposals—on auto rules of origin, a sunset clause, government procurement, and gutting dispute panels seen by the other nations as core to the pact. The moves were long-signaled, as was Canadian and Mexican opposition to them.