Dean, in a speech to Democratic business leaders in West Palm Beach, made the remark in reference to the Harris' handling of the 2000 presidential election recount when she was Florida secretary of state and an honorary chairwoman of George W. Bush's Florida campaign.
Harris certified Bush's 537-vote win in Florida over Democrat Al Gore, putting him in the White House.
Dean said in Wednesday's speech that Democratic incumbent Bill Nelson is "going to beat the pants off Katherine Harris, who didn't understand that it is ethically improper to be the chairman of a campaign and count the votes at the same time. This is not Russia and she is not Stalin."
In a statement Thursday, Harris' campaign said Dean's comments comparing her to a communist dictator responsible for millions of deaths "reflect a lack of understanding and basic decency."
Democratic National Committee spokesman Luis Miranda accused Harris of "trying to deflect attention from her failing campaign and blunders."
Harris holds a commanding lead in the bid for the Republican nomination Sept. 5 against three lesser-known opponents, but she trails Nelson by 37 percentage points, according to a Quinnipiac University poll released Thursday.
On Thursday, she named her fourth campaign manager, an activist in the fight against same-sex marriage in Massachusetts. Bryan Rudnick is the president of Alliance Strategies Group Inc., a Boca Raton-based political consulting firm.