LEXINGTON, Va. — A gymnasium full of college students here on Saturday did what a former Republican Party chairman, a former vice president and a former speaker of the House who addressed their mock convention would not: predict the Republican nominee for president. And they did so with bravado, perhaps characteristic of their nominee, predicting that Donald J. Trump, the business tycoon who has never held elected office, would win the Republican nomination with more than twice as many delegates as the runner-up, Senator Ted Cruz of Texas. For a political culture awash with polls and punditry, the mock convention...