Harris gave up a safe GOP seat in the House to run for the Senate, and was not supported by the national party.
Democrats particularly don't like her because of her role as Florida secretary of state in the 2000 presidential election.
Nelson, casting himself as a moderate, said in their last debate in Orlando that Harris was distorting his record. He said Harris' support for GOP plans to privatize Social Security is "risky," the Miami Herald reported.
The Herald said Harris called Nelson a "flip flopper" who "when he had a chance to raise taxes, he voted for them every time."