Senator Weinberg, a Christie nemesis who watched the news conference from home on Tuesday night, could not pull herself away.

“It almost did me in,” she said. “I found it unpleasant, disconcerting and ….”

She paused. “I am trying to think of negative words that are still appropriate to use.”

She settled on “really awful.”

Those close to Mr. Christie said he had anticipated the furor of his endorsement, but had been a bit taken aback by the depth of the vitriol over the past few days. In his mind, linking arms with Mr. Trump was practical, loyal and politically savvy, in keeping with his decision, in 2011, to become the first major Republican official to endorse Mitt Romney, who ultimately became the nominee.