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Hillary Rodham Clinton holds a strong lead against her Democratic rivals in Iowa despite lingering concerns about her honesty, according to a new poll.

A survey from Suffolk University found that Mrs. Clinton has the support of 54 percent of likely Iowa caucus participants, with Senator Bernie Sanders of Vermont in second place at 20 percent. Vice President Joseph R. Biden Jr., who has not said if he is running, comes in third place with 11 percent.

The poll questioned 500 likely Democratic caucusgoers, and had a margin of sampling error of plus or minus 4 percentage points. It reveals strong loyalty to Mrs. Clinton among Democrats amid concerns about the controversy surrounding her use of a private email server as secretary of state. While most people said that the email problem did not bother them personally, 52 percent said they thought it would damage Mrs. Clinton in a general election.

“There is huge loyalty to her and they are sticking with her,” said David Paleologos, director of the Suffolk University Political Research Center.

Although Mrs. Clinton appears to be the most popular Democrat in Iowa, Mr. Sanders is considered to be the most honest and trustworthy of the party’s candidates and their most likely second choice, edging out Mr. Biden.

Mr. Paleologos said that the support for Mr. Sanders, who has also shown polling strength in New Hampshire, indicates the challenge facing Mr. Biden as he moves closer to deciding about his White House bid. However, he said that the fact that he was not facing ethical questions might make him a more dangerous general election candidate than Mrs. Clinton for Democrats.

“He’s got to find a path to victory in the primary,” Mr. Paleologos said of Mr. Biden. “He would be a better general election candidate because he doesn’t have this email controversy swirling around him.”