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Hillary Rodham Clinton might want to watch her back in Iowa.

Senator Bernie Sanders, of Vermont, who has emerged as her leading rival for the Democratic presidential nomination, is gaining ground in the Hawkeye State, according to a new poll from Quinnipiac University.

The survey of likely Democratic caucusgoers showed Mrs. Clinton leading Mr. Sanders by a margin of 52 percent to 33 percent. While that remains a commanding lead, it is much narrower than a May poll that showed her with 60 percent support and him with 15 percent.

“Secretary Hillary Clinton should not be biting her fingernails over her situation in the Iowa caucus, but her lead is slipping and Senator Bernie Sanders is making progress against her,” said Peter A. Brown, assistant director of Quinnipiac’s poll.

It is the first time that Mrs. Clinton has fallen below 60 percent in Iowa this cycle, reflecting Mr. Sanders’ emergence as an increasingly credible threat to her candidacy. Vice President Joseph R. Biden Jr., who has not decided if he will run for president, trails them at 7 percent, former Gov. Martin O’Malley of Maryland follows at 3 percent and former Senator Jim Webb of Virginia is at the back of the pack with 1 percent.

Thursday’s poll also reflects momentum that has been apparent on the ground in Iowa. Mr. Sanders has attracted large crowds at his rallies across the state and his message of raising taxes on the rich and cutting military spending is resonating with Democratic activists there.

While the results could be concerning to Mrs. Clinton, who finished in third place in the Iowa caucuses in 2008, there were some other encouraging signs. Despite questions about her honesty in some national polls, potential caucusgoers said they had confidence in her character and her leadership skills.

“By overwhelming margins, likely Democratic caucusgoers themselves consider her to be honest; believe she understands the problems of average folks, and is a strong leader,” Mr. Brown said.