Story highlights Bernie Sanders is unhappy with the "tone" of the Clinton campaign's attacks against him

He is trailing in the latest Des Moines Register/Bloomberg Politics poll, 42% to 45%

Charles City, Iowa (CNN) Bernie Sanders rallied supporters in Iowa with political red meat Saturday, highlighting his ideological purity on issues of concern to liberals and saying he would not let Hillary Clinton "distort" his record.

"I am disturbed, I have to say, (that) the tone of Secretary Clinton's campaign is such that they are just bringing forth a lot of inaccurate statements regarding what I believe," Sanders told canvassers in Charles City.

Saturday night in Cedar Rapids, he really let it fly.

"I am disappointed by the tone of her campaign," he said, citing recent Clinton attacks, including criticism of him calling Planned Parenthood part of the "establishment" and repeated jabs over his gun control record.

"My opponent has announced that she has raised tens of thousands of dollars from her super PAC for special interests," Sanders said. "Let me tell you proudly, I have raised zero dollars."

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