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 at Washington and Lee University offers one of the most meticulous and intriguing predictions of its kind, having successfully forecast the nominee of the party not in the White House in 19 of 25 attempts, including all but two since 1948.

Despite the convention’s decisive outcome, its political analysts said they had not been immune to the difficulty posed by the breakdown in traditional Republican voting patterns seen in the campaign so far.