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Nearly two days into an outcry over Illinois Sen. Mark Kirk mocking his Democratic opponent's ethnic heritage, the LGBT rights group Human Rights Campaign is withdrawing its endorsement.

It was the first time in the group's 36-year history that it was withdrawn an endorsement, HRC President Chad Griffin wrote in a Medium post Saturday afternoon.

"Senator Kirk’s comments about his opponent’s heritage were deeply offensive and racist," Griffin wrote. "His attempt to use Congresswoman Tammy Duckworth’s race as a means to undermine her family’s American heritage and patriotism is beyond reprehensible."

Duckworth is a veteran of the Iraq war who was born in Thailand to a Thai mother of Chinese descent and an American father who served in World War II.

During Thursday night's Senate debate, Duckworth said, "My family has served this nation in uniform, going back to the Revolution. I'm a daughter of the American Revolution. I've bled for this nation … Families like mine are the ones that bleed first."

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Kirk responded: "I had forgotten that your parents came all the way from Thailand to serve George Washington."

Initially, Kirk declined to apologize, with his campaign instead issuing a statement that sought to change the subject: "Senator Kirk has consistently called Rep. Duckworth a war hero and honors her family's service to this country. But that's not what this debate was about. Rep. Duckworth lied about her legal troubles, was unable to defend her failures at the VA and then falsely attacked Senator Kirk over his record on supporting gay rights."

At first, the HRC said its endorsement stood, but a spokesman called the comments "wrong and inappropriate" and asked him to "rescind his comments immediately."

On Friday Kirk did apologize.

Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family's service. #ilsen — Mark Kirk (@MarkKirk) October 28, 2016

In Saturday's post, Griffin said the apology wasn't enough: "Yesterday, Senator Kirk tweeted an apology that failed to adequately address the real harm and magnitude of his words."

Gay rights advocates had already been questioning why the influential organization had sided with Kirk over Duckworth, given that she has a more consistent pro-LGBT track record.

Griffin cited Kirk's 100 percent score on HRC's most recent congressional scorecard, but his score was significantly lower in prior years. Duckworth has consistently maintained a perfect score.

"It’s vitally important that we continue to build bipartisan coalitions so that we may continue to move equality forward," wrote Griffin by way of explanation. "But," he added, "Events this week have gone beyond the pale for our standards of leadership."

Kirk campaign spokesperson Eleni Demertzis in a statement to NBC News Saturday called the Human Rights Campaign decision unfortunate.

"Senator Kirk is incredibly proud of his leadership to fight discrimination at every level, including speaking on the steps of the Supreme Court to urge them to support marriage equality," Demertzis said. "The HRC decision today is unfortunate because it was based on politics instead of reality and further exemplified just how uncomfortable HRC was in supporting a Republican who was a leader for their efforts, including eradicating discrimination."

In June, Kirk became one of the first Republican senators to un-endorse Donald Trump amid the candidate's headline-generating controversies, and later called Trump "too bigoted and racist" to be president.

Kirk is considered one of the most vulnerable GOP senators in this election cycle, and the race could play a role in whether Democrats retake control. Republicans hold a majority in the Senate with 54 seats, but must defend seats in six states where President Barack Obama won in 2012, including Illinois.