Hillary Clinton enjoys a 19-point lead among likely Democratic caucus-goers in the key state of Iowa over her nearest challenger, Vermont Senator Bernie Sanders, but that advantage has shrunk 26 points since May, according to a Quinnipiac University poll released Thursday.

The former secretary of state gets the support of 52 percent of her party's likely caucus-goers in the state, which holds the nation's first nominating contest, while Sanders, a Senate independent and self-described socialist seeking the Democratic nomination, pulls in support from 33 percent. In May, the split was 60 percent to 15 percent.

It is the first time Clinton has received less than 60 percent support in the poll, according to assistant poll director Peter A. Brown.

Senator Bernie Sanders speaks during a campaign rally in Madison, Wisconsin, on July 1, 2015.
Senator Bernie Sanders speaks during a campaign rally in Madison, Wisconsin, on July 1, 2015.
Photographer: Christopher Dilts/Bloomberg

A Bloomberg Politics poll conducted June 19-22 found Clinton the first choice of 50 percent of likely Iowa Democratic caucus participants, with 24 percent favoring Sanders.

Vice President Joe Biden, who has not said he is running, received 7 percent. Former Maryland Governor Martin O'Malley drew 3 percent, and undeclared former Virginia Senator Jim Webb got support from 1 percent. Former Rhode Island Governor Lincoln Chafee received no support.

“Secretary Hillary Clinton should not be biting her fingernails over her situation in the Iowa caucus, but her lead is slipping,” said Brown in a news release. But Sanders, he said, “certainly can’t be ignored, especially with seven months until the actual voting.”

Also on Bloomberg Politics: Inside the Grassroots Group That Wants America to Feel the Bern

The most welcome news for Clinton may be the responses about her character, with 85 percent saying they had a favorable opinion of her and 83 percent saying they believe “Clinton cares about the needs and problem of people like” themselves—figures that have remained stable since the last poll.

The poll of 761 likely Iowa Democratic caucus participants was conducted June 20-29. The margin or error is plus or minus 3.6 percentage points.

Before it's here, it's on the Bloomberg Terminal. LEARN MORE