Mr. Paul is the latest in a series of candidates whose quick improvement in polls has drawn new scrutiny of the more problematic portions of their résumés. The focus on his newsletters comes as he seeks to seize momentum in polls by raising questions about his opponents.

During an appearance on “The Tonight Show With Jay Leno” on Friday, Mr. Paul joked that Representative Michele Bachmann of Minnesota, a Republican rival for the nomination, “hates Muslims, she wants to go get them.” He also concurred with Mr. Leno that former Senator Rick Santorum speaks about “gay people” almost exclusively, adding, “And Muslims.”

Though Mr. Kesari said those comments were intended to be “lighthearted,” they drew criticism from some commentators, including the Fox News host Greg Gutfeld, who on Monday pointed to Mr. Paul’s newsletters as evidence that he was being hypocritical.

Mr. Paul has survived previous questions about his newsletters. During his 1996 race for the House, Democrats publicized issues of his newsletter that called Barbara Jordan, the African-American Texas congresswoman, a “half-educated victimologist” and said of crime in Washington, D.C., “I think we can safely assume that 95 percent of black males in that city are semi-criminal or entirely criminal.”

He defended the statements to The Dallas Morning News at the time, saying they were taken out of context. He also told the newspaper he did not know that his newsletter — with 7,000 to 8,000 subscribers — was listed by a neo-Nazi group called Heritage Front, apparently as recommended reading, under the Internet heading “Racialists and Freedom Fighters.”

But in an interview in 2001 with Texas Monthly, Mr. Paul said he regretted that he had not admitted that he had not written the newsletters. “They were never my words, but I had some moral responsibility for them,” Mr. Paul said. He said that he had “actually really wanted to try to explain that it doesn’t come from me directly,” but that his campaign aides had told him, “Your name was on that letter and therefore you have to live with it.”

The question now is whether the newsletters’ reappearance will hurt Mr. Paul with some of his younger supporters. Jake Mescher, a freshman organizer for Mr. Paul at Drake University in Des Moines, predicted that the newsletters would not reduce the ardor of his supporters. “I’ve heard of that four or five times, but it really has not made me wary,” Mr. Mescher said. “He has nothing in his record to suggest that that is part of his personal beliefs as a member of Congress or a presidential candidate.”