Democratic U.S. Senate candidate John Hickenlooper on Monday apologized for comparing the plight of elected officials to slaves pulling the oars in a slave ship during remarks the former governor delivered years ago at a charity event.

Hickenlooper’s quip comparing the relationship between politicians and their schedulers to galley slaves and their whip-wielding slave drivers surfaced Monday when Tay Anderson, a director on the Denver Public Schools board, posted a grainy video clip of Hickenlooper’s remarks to social media, calling the equation “utterly disgusting.”

“Well once that happens then the schedulers, who really, if I was to describe a scheduler, a political scheduler – imagine an ancient slave ship with the guy with the whip, and you’re rowing – we elected officials are the ones that are rowing,” Hickenlooper says to laughter in the 24-second video clip, recorded by a political tracker at a June 3, 2014, Make-a-Wish Colorado event.

“@Hickenlooper you have some explaining to do,” Anderson said in a tweet attached to the video. “[R]eferencing my ancestors pain of being brought over here in chains to a political scheduler is utterly disgusting.”

Anderson, who has emerged as a leader of the Black Lives Matter protest movement in Denver, has endorsed Hickenlooper’s Democratic opponent, former state House Speaker Andrew Romanoff, in the June 30 primary for the seat held by Republican U.S. Sen. Cory Gardner.

“Taking a look at this video from six years ago, I recognize that my comments were painful. I did not intend them to be. I offer my deepest apologies,” Hickenlooper said in a statement provided by a campaign spokesman.

On Friday, the Hickenlooper campaign’s press staff told Colorado Politics they were unaware of his “slave ship” remarks.

Anderson rejected Hickenlooper’s apology, asking if the former governor will “commit to going through an equity and implicit bias training?”

“[A]n apology isn’t good enough for me, because it should’ve been common sense not say things like that,” Anderson tweeted Monday afternoon.

Hickenlooper has taken heat in recent weeks for an answer during an online candidates’ forum on racial justice. “‘Black Lives Matter’ means every life matters,” Hickenlooper said, echoing a formulation activists say ignores America’s systemic racism.

In a televised debate with Romanoff last Monday, Hickenlooper said he “stepped on” his words and later pointed to policing reforms he put in place in Denver when he was mayor.

During his years in politics – including a run for president last year – Hickenlooper has won a reputation for putting his foot in his mouth in a cavalcade of misstatements sometimes called “Hickenbloopers.”

Ronna McDaniel, the chairwoman of the Republican National Committee, however, didn’t cut the Democrat any slack Monday when his 2014 remarks showed up on social media.

“Democrat John Hickenlooper offensively likens his relationship with his scheduler to that of a slave & slave driver. Schumer & Dem leaders need to condemn this.” McDaniel said in a tweet linked to the video clip, referring to Senate Minority Leader Chuck Schumer, who recruited Hickenlooper into the Senate primary.

“From this to his massive ethics controversy, it’s clear Colorado deserves better.”