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Beyond the primary battle, the surge of negative advertising could start to harm Mr. Trump’s general election prospects.

It started in earnest in December, when Right to Rise, a group supporting Jeb Bush, spent $2.1 million on an ad portraying Mr. Trump as a “bully.” At the time, Mr. Trump had relatively high unfavorable ratings compared with the other candidates: 57 percent among all Americans, according to a CNN/ORC poll from that month.

But as the ads began to increase in frequency and the tone turned more negative, his national unfavorable rating began to climb unabated. A CNN/ORC poll last month found that it was up 10 points from December, to 67 percent: 11 points higher than any other candidate still in the race, Republican or Democrat.

Mike Murphy, who was the chief strategist for Right to Rise, said the ads had kept Mr. Trump from consolidating more support in the Republican primary.

“I think the negative ads are having a chilling effect on his ability to grow his natural share with converts,” Mr. Murphy said. However, he added, Mr. Trump’s own mouth plays a role.

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“He creates an environment where negative ads are a little more fertile,” Mr. Murphy said, adding that the ads were having more impact now that fewer candidates are in the primary race.

Larry McCarthy, a Republican strategist who worked with Mr. Murphy and now produces ads for Our Principles PAC, said his group had found, through polling and focus groups, that the best way to go after Mr. Trump was to use the bombastic billionaire’s own words against him.

“Campaigns use negative ads because they work,” said Ryan Williams, a Republican strategist and vocal critic of Mr. Trump. “And Donald Trump has provided his opponents with a bonanza of negative information to use in ads targeting his candidacy.”

Compounding the risks for Mr. Trump is that he has done little to counteract the effort with his own spending. He has spent just over $16 million on television ads, and in later contests, he went on the air late or not at all. A spokeswoman for Mr. Trump, Hope Hicks, did not respond to an email requesting comment.

Mr. Williams, who worked on Mitt Romney’s 2012 presidential campaign, recalled how allies of President Obama aired a devastating ad that eviscerated Mr. Romney’s character. It was paired with the Obama campaign’s own ads, which began running in the late spring and stayed on the air through the election.