WASHINGTON -- Sen. Bernie Sanders on Thursday apologized to women who have shared experiences of harassment by male supervisors while working on his 2016 presidential campaign, a controversy that has raised fresh questions about his potential second run in 2020.

The Vermont independent said on Capitol Hill that the alleged misconduct by male aides "was absolutely unacceptable and certainly not what a progressive campaign or any campaign should be about." He said that rhetoric about "ending sexism and ending all forms of discrimination ... cannot just be words."

"To women in our campaign who were harassed or mistreated, I apologize," Sanders said.

Politico reported Wednesday that a former Sanders adviser, Robert Becker, had forcibly kissed a female subordinate in mid-2016. Becker denies the allegation.

Sanders said he was unaware of the alleged misconduct by Becker, who has recently taken unofficial travel in a bid to bolster a possible 2020 presidential campaign by the senator. Sanders also said he wasn't aware of a reported $30,000 settlement to resolve a discrimination claim against his 2016 campaign.

The New York Times has also reported on multiple claims of mistreatment against female Sanders campaign aides by senior male officials.

In his initial response this month to female campaign aides who had come forward about their harassment, Sanders told CNN he didn't know about the allegations in 2016 because "I was little bit busy running around the country trying to make the case." He projected more contrition on Thursday, saying that "our standards, our procedures, our safeguards, were clearly inadequate." Sanders said his 2018 Senate re-election campaign instituted "some of the strongest sexual harassment policies in the country."

The emergence of the misconduct claims against Becker and other male Sanders aides has cast a pall over his preparations for the 2020 campaign, which could pit the 77-year-old senator against a half-dozen or more of his congressional colleagues in a hotly contested Democratic primary for the nomination to take on President Donald Trump.