At a town hall meeting in Portsmouth, N.H., Mr. Christie expressed disbelief at Mr. Trump’s comment Saturday that he could shoot someone on Fifth Avenue in Manhattan and lose no support in the 2016 campaign.

“It’s pretty amazing to say it, isn’t it?” Mr. Christie asked.

Briefly slipping into an impression of Mr. Trump — “We’re stupid,” he deadpanned in a throaty baritone — Mr. Christie said he empathized with voters who want to “burn Washington down.”

“But who’s going to rebuild it once it gets burned down?” Mr. Christie said. “That’s what you’ve got to think about.”

By nominating the wrong candidate, he continued, “We could wind up turning over the White House to Hillary Clinton for four more years.”

That prospect, he said, would be “like the eight years we’ve just had, except worse.”