WASHINGTON—A $1.75 trillion Democratic spending proposal could give a big boost to electric vehicles especially to Detroit’s Big Three automakers and the U.S. Postal Service, but it faces opposition from Republicans, foreign automakers, Canada and Mexico. The House plan boosts electric vehicle credits to up to $12,500 per vehicle, including $4,500 for union-made vehicles and $500 for U.S.-made batteries. Vehicles would have to be made in the United States starting in 2027 to qualify for any credit. The EV tax credits would cost $15.6 billion over 10 years and disproportionately benefit Detroit’s Big Three automakers—General Motors, Ford Motor, and Chrysler-parent Stellantis NV—which assemble their U.S.-made vehicles in union-represented plants. United Auto Workers (UAW) President Ray Curry said the provision will “create and preserve tens of thousands of UAW members’ jobs,” and “would be a win for auto manufacturing workers.” “The framework announced today supports good paying union jobs and stands …