Momentum is growing among President Trump’s advisers to place 25% tariffs on Mexico and Canada as soon as Saturday, bucking conventional wisdom in Washington and on Wall Street that he would back off the threatened levies as he has in the past in exchange for concessions.

Every day a flurry of parts and products are shuttled between Texas and Mexico, part of a cross-border supply chain to make everything from cars and heavy duty machinery to flat screen televisions. The future of that partnership,