In fact, Mr. Bush was in Exeter for one of four town-hall-style events he had scheduled on a single Saturday, a feat intended as much to demonstrate stamina and resolve as to get him in front of as many voters as possible before the holiday break.

He is also trying to show his potential by being the first Republican candidate to consistently attack Mr. Trump, calling him a “jerk” unfit for the presidency. Mr. Bush’s campaign and Right to Rise, the “super PAC” supporting him, have begun highlighting in television ads and online videos the idea that Mr. Bush is the only candidate strong enough to stand up to Mr. Trump.

Mr. Bush’s team rejects the notion that Mr. Trump’s attack has devastated their candidate, saying the insult is old news. “Donald is a lazy candidate with so little interest in the job that he gets his national security information from ‘the shows,’” said Tim Miller, a Bush spokesman. “Jeb is going to continue to outwork him all the way to the White House.”…

Frank Luntz, a Republican pollster, conducted a focus group of voters during the most recent Republican debate in Las Vegas, in which Mr. Bush performed well during a feisty exchange with Mr. Trump. When Mr. Bush and Mr. Trump began to tussle, Mr. Luntz said, the participants in the focus group started “laughing hysterically” and saying, “Oh, there’s the low-energy guy.”