WASHINGTON—Toyota Motor Corp. and Tesla clashed with Ford Motor Co. and the United Auto Workers (UAW) union over a proposal by Democrats in the U.S. House to give union-made, U.S.–built electric vehicles an additional $4,500 tax incentive. In a letter to Congress, Toyota said the plan discriminates against nearly half of American autoworkers who do not belong to a union and called on lawmakers to reject giving “exorbitant tax breaks” to wealthy buyers of high-priced cars and trucks. The bill, set to be taken up on Tuesday by the House Ways and Means Committee as part of a proposed $3.5 trillion spending bill, would benefit Detroit’s Big Three automakers—General Motors, Ford Motor Co and Stellantis NV, the parent of Chrysler—which assemble their U.S.–made vehicles in UAW-represented plants. The proposal boosts the maximum tax credit for these electric vehicles to $12,500—including a $500 credit for using U.S.–made batteries—from the current $7,500, …