Nikki Haley, the former South Carolina governor seeking the Republican Party’s presidential nomination, appears to have weathered a holiday-season gaffe on the causes of the Civil War, but the controversy over her answer, which neglected to mention slavery, was a gift to a rival, former Gov. Chris Christie of New Jersey.

And that fresh ammunition may be the most lasting fallout for her effort to catch former President Donald J. Trump in the nation’s first Republican primary in New Hampshire on Jan. 23.

With less than two weeks before the Iowa caucuses, Ms. Haley is expected back in southern New Hampshire on Tuesday for a two-day campaign swing, working to maintain the momentum that has lifted her to second place in the state. But the final week of 2023 was a particularly rocky one. She flubbed the name of the Iowa Hawkeyes’ star basketball player Caitlin Clark, stirred anger and frustration among the independent and moderate factions of her base over her Civil War answer at a Berlin, N.H., town hall meeting, then potentially provoked the anti-Trump faction again when she said she would pardon Mr. Trump should he be convicted.

Mr. Christie, who will be in the state on Thursday and Friday, has seized on Ms. Haley’s gaffe, and both of their campaigns are at a pivotal moment. They have long been on a collision course in New Hampshire, which Mr. Christie has made his do-or-die state and where Ms. Haley has been climbing.

A New Hampshire poll in November by Emerson College Polling and the Boston television station WHDH found Mr. Trump with the support of 49 percent of Republican primary voters, consistent with his numbers in August. Ms. Haley had climbed 14 percentage points, drawing the support of 18 percent of voters, while Mr. Christie followed with a steady 9 percent.

“The first reason given in the secession notice from the South Carolina government at the time the Civil War began was because the North opposed the expansion of slavery to the Western territories,” Mr. Christie said on CNN on Friday. “Now Nikki knows all that, and she’s not saying it because she’s afraid to say it.

Polling averages in New Hampshire do not suggest Mr. Christie is threatening Ms. Haley, much less Mr. Trump, but his third-place position, with about 11 percent of the Republican primary electorate, is a damper on Ms. Haley, whose rise in the state appears to have stalled.

Both are competing for the same voters: Republicans who have soured on Mr. Trump and independents motivated to vote in the Republican primary to thwart the former president’s aspiration to reclaim the White House. On Tuesday, a Trump campaign memo called that cohort “a ‘Coalition of the unwilling’” that both candidates hope “is enough to slow President Trump down.”

The combined support of Ms. Haley and Mr. Christie would put one of them within striking distance of the front-runner, though the math is not that simple. Both candidates most likely have voters who would go elsewhere or sit out the election if their preferred candidate dropped out.

“I think he should stay in — keep the heat up,” Doug Houghton, 71, a retired airline pilot and independent voter, said of Mr. Christie in late November at a Haley town hall in Hooksett, N.H.

Furthermore, the Civil War snafu seems to have squelched any talk of the New Jerseyan throwing his support to the South Carolinian.

In interviews with more than a dozen anti-Trump Republicans, independents and centrist Democrats at Haley campaign events, several described Mr. Christie as a sort of moral compass for the primary field and the only candidate who could keep Ms. Haley from straying too far to the right.

Christie campaign aides say Ms. Haley’s suggestion that the Civil War was fought over “the freedoms of what people could and couldn’t do” was not a one-off. In 2010, she said the war was over “tradition versus change.” The issue, they said, goes to the foundation of Mr. Christie’s campaign — that only he has the fortitude to challenge the Republican electorate to put the divisiveness of the Trump era behind the party. They pointed to her long, convoluted answer last week when asked whether she would be Mr. Trump’s running mate. Over a rambling two minutes, she failed to answer.

“What it’s done is exposed something about Governor Haley that’s existed throughout the campaign but never so starkly — an unwillingness to speak the truth about difficult subjects, and the most difficult subject in the Republican primary is Donald Trump,” Michael DuHaime, a longtime Christie confidante and campaign adviser, said of the Civil War comments.

Ms. Haley notched a significant political victory last month when Chris Sununu, New Hampshire’s popular governor, threw his support behind her after long weighing whether to endorse Ms. Haley, Mr. Christie or Gov. Ron DeSantis of Florida. Mr. DuHaime conceded that the endorsement put some distance between Ms. Haley and Mr. Christie, though not nearly enough for Ms. Haley to catch the front-runner.

In national television interviews and campaign appearances, Mr. Christie has consistently criticized Ms. Haley over what he describes as her unwillingness to take on Mr. Trump — “Either run against him or don’t run against him” — and her stance on abortion, which he argues Ms. Haley changes to sound more hard-line in Iowa and less so in New Hampshire. (Ms. Haley has rejected this characterization at her own town halls.)

The two have been cordial with each other on the national debate stage, but Mr. Christie has also been accused by Ms. Haley’s supporters of playing into gender tropes. He has referred to Mr. Sununu as Ms. Haley’s “political husband.” In the fourth presidential debate, he himself came to her defense against her rivals as she appeared to retreat into the background.

So far, the Haley campaign apparatus has not paid much attention to Mr. Christie. A new ad by the super PAC aligned with her, SFA Fund, attacked Mr. DeSantis, not Mr. Christie or Mr. Trump, indicating an intention to establish Ms. Haley as the clear second-place candidate ahead of the Iowa caucuses on Jan. 15.