If Donald Trump makes good on his threat to slap 25% tariffs on everything imported from Mexico and Canada, the price increases that could follow will collide with his campaign promise to give American families a break from inflation.

The tariffs, if implemented, could dramatically raise prices for American consumers on everything from gas to automobiles to agricultural products.

WASHINGTON—President-elect Donald Trump pledged that soon after taking office he will slap steep tariffs on Mexico and Canada, two of America’s closest allies, as well as China, the clearest indication since his election victory that he plans to follow through on the tough campaign rhetoric that helped propel him to the White House.