Former Gov. Chris Christie of New Jersey said in a new ad released Thursday that he had been wrong to endorse Donald J. Trump in 2016.

“I have an admission to make,” Mr. Christie said directly to the camera in the 60-second ad, part of a series his campaign is running in New Hampshire, where he has staked his campaign on a strong showing. “Eight years ago, when I decided to endorse Donald Trump for president, I did it because he was winning, and I did it because I thought I could make him a better candidate and a better president. Well, I was wrong. I made a mistake.”

Mr. Christie endorsed Mr. Trump in 2016 after ending his own presidential campaign, and he went on to be a powerful surrogate. His campaign argues that, while several Republicans who previously supported Mr. Trump are now running against him for the party’s nomination, Mr. Christie is the only one willing to say he erred by supporting Mr. Trump in the first place. He suggested in the ad that this was evidence of his “character.”

“Now, we’re confronted with the very same choice again,” he said in the ad, which was first reported by Axios. “Donald Trump is ahead in the polls, so everyone says, ‘Anyone who’s behind him should drop out, and we should make our choice Donald Trump versus Joe Biden.’ Well, Joe Biden has had the wrong policies, and Donald Trump would sell the soul of this country. Neither choice is acceptable to me, and it shouldn’t be acceptable to you.”

Over the past few weeks, an increasing number of Republicans — most prominently Gov. Chris Sununu of New Hampshire — have called on Mr. Christie to end his campaign. Their argument is not for a rematch between Mr. Trump and Mr. Biden, but rather for narrowing the field of Mr. Trump’s primary opponents to a single candidate who can defeat him and prevent such a rematch.

Mr. Sununu and many Republican donors and strategists think that candidate should be Nikki Haley, the former South Carolina governor.

Mr. Christie is polling better in New Hampshire than in other early-voting states — he is averaging about 11 percent there, good enough for third place ahead of Gov. Ron DeSantis of Florida — but he is still far behind Ms. Haley, who is herself far behind Mr. Trump. Mr. Christie says Ms. Haley is not sufficiently distinguishing herself from the former president.

Mr. Sununu’s endorsement of Ms. Haley last month was a blow to the Christie campaign. He followed it a couple of weeks later by declaring Mr. Christie’s bid to be “at an absolute dead end” and suggesting that his continued presence in the race would only help Mr. Trump.