Donald Trump says he will force Mexico to pay for a border wall by threatening to cut off the flow of billions of dollars in payments that immigrants send home to the country, an idea that could decimate the Mexican economy and set up an unprecedented showdown between the United States and a key diplomatic ally. Trump outlined how he would seek to force Mexico to pay for his 1,000-mile border fence, which Trump has made a cornerstone of his presidential campaign. But the feasibility of Trump’s plan is unclear both legally and politically, and also would test the bounds of a president’s executive powers in seeking to pressure another country.