Washington (AFP) - Bernie Sanders apologized Thursday for "unacceptable" sexual harassment that occurred on his 2016 presidential campaign, directly addressing an issue that could weigh on the senator's potential 2020 White House bid.

Sanders has been on the defensive for weeks while he mulls another presidential bid, as several former campaign staffers have made allegations of sexual misconduct and pay disparity.

"It now appears that as part of our campaign there were some women who were harassed or mistreated. I thank them, from the bottom of my heart, for speaking out," Sanders said in a statement.

Sanders does not stand accused of harassing staffers himself, but has been criticized for poorly handling the complaints.

The harassment allegations "speak to unacceptable behavior that must not be tolerated in any campaign or any workplace," Sanders said.

"To the women in that campaign who were harassed or mistreated I apologize. Our standards and safeguards were inadequate."

Sanders had apologized before, albeit not in such a comprehensive fashion.

Thursday's statement emerged hours after a Politico story that detailed new accusations against a top Sanders adviser in 2016 who is already taking steps towards working on a possible 2020 Sanders campaign in early voting states like Iowa.

An unnamed woman accused the aide, Robert Becker, of forcibly kissing her shortly after the Democratic National Convention in July 2016.

"This can't happen in 2020," the woman said in Politico. "You can't run for president of the United States unless you acknowledge that every campaign demands a safe work environment for every employee and volunteer."

The Sanders situation hit headlines after November's midterm elections, in which the #MeToo women's movement -- propelled by female voters' frustration with President Donald Trump, who himself stands accused of assault or harassment by multiple women -- helped sweep Democrats to victory.

But that same anti-harassment drive that recently brought many men to account for their sexist behavior may ensnare some Democratic potential 2020 candidates.

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They include former vice president Joe Biden, who has acknowledged shortcomings in handling the 1991 hearings in which Anita Hill, a former aide to Supreme Court nominee Clarence Thomas whom she accused of misconduct, testified before an all-male, all-white Senate committee.

And Senator Cory Booker admitted years ago that he groped a girl while they were teenagers. He has said the experienced changed him and that he learned to be a better person.

Sanders, 77, is an independent who ran for the Democratic nomination in 2016.