DALLAS — President Donald Trump delivered the keynote speech on day one of the GOP's inaugural midterm convention, alternating between touting his successes in office and rallying thousands of supporters for Republican candidates ahead of November's general election before making a promise to American voters.In a stunning move, Trump made a promise unheard of in the history of American politics — offering a cash prize should Republicans keep control of Congress."I can make this promise to you, and here is my promise," he said. "If the Republicans win the House of Representatives and the United...