Donald Trump said his ambitious package of across-the-board tax reductions is meant to serve as a starting point for negotiations with Congress, and that he could see taxes actually rise for upper-income households if needed to win cuts for the middle class.

“I’ll tell you what the real concept is,” the presumptive Republican presidential nominee said in an ABC interview that aired Sunday. “Lower taxes for business, lower taxes for the middle class, lower taxes for everybody and then we’re going to start negotiating.” On tax...