PHILADELPHIA — President Trump plans to make Mexico pay for his border wall by imposing a 20 percent tax on all imports into the United States from Mexico, raising billions of dollars that would cover the cost of the new barrier. The proposal, which Sean Spicer, the White House press secretary, said the president discussed privately with congressional Republicans before giving remarks at a party retreat here, would be a major new economic proposal that could have far-reaching implications for consumers, manufacturers and relations between the two governments. Mr. Spicer said the 20 percent tax on annual Mexican imports would...