In rigorous practice sessions that have crisscrossed the country in the past few weeks, Mr. Bush has been preparing pointed responses to Mr. Trump’s most frequent insults (he has called Mr. Bush “low energy” and a slave to donors) and crafting lines of attack on Mr. Trump’s history of liberal statements and unconventional policy plans, according to those who have been told of the plans.

Hey, we did our best. Look at what we had to work with!

Interviews with current and former advisers to Mr. Bush and reviews of video footage from his debates as a candidate for governor reveal him to be a debater with as many deficits as strengths. On stage, his mastery of policy is almost unrivaled, but he struggles to synthesize it in compelling ways.