Nelson put out a statement late Wednesday that said "sexual harassment is never acceptable" and that he agreed with other Democratic senators that Franken "should step aside."
His move came several hours after a majority of the Senate's Democrats, including more than a half-dozen women, called on Franken to resign after another women emerged saying he forcibly tried to kiss her in 2006. That brought to at least seven the number of women accusing him of sexual impropriety.
Franken is expected to make an announcement on Thursday.
Nelson is seeking a fourth term in office in 2018 but is expected to be challenged by Florida Gov. Rick Scott.
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