Story highlights Sanders, the fiery progressive senator from Vermont, trails Clinton 37% to 30%.

Clinton has lost one-third of her supporters since May.

Washington (CNN) Bernie Sanders continues to cut into Hillary Clinton's once-commanding lead among Iowa Democrats, closing to just 7 points of the party front-runner in the first-in-the-nation caucus state, a new poll has found.

A survey released late Saturday afternoon by the Des Moines Register and Bloomberg Politics finds that Sanders, the fiery progressive senator from Vermont, trails Clinton 37% to 30%. The former secretary of state has lost one-third of her supporters since May.

Sanders' support owes more to voters' enthusiasm for his candidacy than opposition to Clinton, the poll found. A whopping 96% of his backers say they support him and his ideas, with just 2% saying their vote is motivated by a desire to stop a Clinton candidacy. As for the controversy surrounding Clinton's use of email while leading the State Department, 61% of likely Democratic caucusgoers say the issue is not important to them.

Sanders has a deeper reservoir of support, the poll found. Thirty-nine percent of likely caucusgoers say their feelings about Sanders are very favorable, with just 8% having a negative view of him. That's a sharp contrast to Clinton: 27% view her very favorably, but 19% view her negatively.

Saturday's poll marks a remarkable eight-month climb for the self-proclaimed Democratic socialist from Vermont, who is garnering support in part from his anti-establishment rhetoric. Back in January, half of likely Democratic caucusgoers were unfamiliar with Sanders, and he was pulling in just 5% of support.

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