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On Tuesday, Donald J. Trump’s campaign started running a negative ad in South Carolina against Senator Ted Cruz that was searing, describing the winner of the Iowa Republican caucuses as a fake and a “Washington insider.”

The spot also scolded Mr. Cruz for his campaign’s role in spreading claims that Ben Carson had suspended his campaign while the caucuses were taking place; Mr. Carson, in fact, had not ended his candidacy, and Mr. Cruz later apologized to him.

But on Wednesday the spot was abruptly pulled down by the campaign. The Wall Street Journal reported on the strategic shift, saying that Mr. Trump’s campaign manager, Corey Lewandowski, had made the decision to go with all positive spots beginning on Thursday. Mr. Lewandowski made the decision after seeing that Mr. Trump’s more positive-focused tone in New Hampshire appeared to help him at the end, according to The Journal.

Asked to confirm the Journal’s account about only running positive spots going forward, Hope Hicks, a spokeswoman for Mr. Trump, said that is now the campaign’s plan, although she did not elaborate on the reason for the change.

But just as Mr. Trump’s ads came down, Mr. Cruz began airing a spot that had initially been available only online, featuring children playing with a Trump action figure and smashing a doll house while saying “eminent domain.” During the last debate, Mr. Trump was forced to defend the use of eminent domain in one of his Atlantic City casino projects.

In the final days of the New Hampshire race, Mr. Trump was less caustic about his rivals than he had been before, although he couldn’t resist repeating a vulgarity that someone in the crowd at his final rally used about Mr. Cruz.

But South Carolina’s Republican primaries have historically turned into bare-knuckled fights, and whether Mr. Trump changes course again, especially now that Mr. Cruz has aired an ad against him, remains to be seen.