“He will have a very easy time doing whatever he can do through executive authority alone. That includes the conduct of foreign relations, up to a point,” said William Galston of Washington’s Brookings Institution, who worked in the White House under President Clinton.

By contrast, “he would have a hard time doing anything that requires the cooperation of Congress.”

If elected, Trump would take office after what amounts to a hostile takeover of the Republican Party and over the opposition of Democrats. He probably would not be able to count on much support from either side on Capitol Hill. That would mean trouble for his promises to build a wall along the Mexican border or to round up and deport the roughly 11 million people currently in the U.S. without legal authorization. Both would require Congress to approve billions of dollars in new appropriations even if Trump could pressure the Mexican government into reimbursing the U.S. for the cost of the wall, which Mexico says it won’t consider.

On foreign policy, a President Trump would face a different set of constraints — other countries.