The Human Rights Campaign withdrew its endorsement of Sen. Mark Kirk Mark Steven KirkLiberal veterans group urges Biden to name Duckworth VP On the Trail: Senate GOP hopefuls tie themselves to Trump Biden campaign releases video to explain 'what really happened in Ukraine' MORE (R-Ill.) on Saturday after he questioned his opponent Rep. Tammy Duckworth's (D-Ill.) American heritage during a debate.

"On Thursday night, Senator Kirk’s comments about his opponent’s heritage were deeply offensive and racist," the group's president, Chad Griffin, wrote in an open letter posted to Medium.

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"His attempt to use Congresswoman Tammy Duckworth’s race as a means to undermine her family’s American heritage and patriotism is beyond reprehensible," Griffin added.

Griffin said it was the first time in the LGBT advocacy group's 36-year history that it had revoked an endorsement.

"The diversity of our movement is our greatest strength, and Senator Kirk’s remarks were an affront to our most fundamental values," the statement read. "We have therefore voted to endorse Congresswoman Tammy Duckworth, who has been a strong LGBTQ ally in the House of Representatives, and HRC will contribute the maximum amount to her campaign," he wrote.

During a segment on Middle East foreign policy during the debate, Duckworth detailed her family's long history of military service going back to the Revolutionary War.

"I forgot that your parents came all the way from Thailand to serve George Washington," Kirk responded.

Kirk, considered one of the most vulnerable GOP senators running for reelection, apologized to Duckworth on Friday morning.

"Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family's service," Kirk wrote on Twitter.

In an email to The Hill, Kirk's campaign emphasized the Senator's outspoken support for marriage equality.

​"​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," said Eleni Demertzis, the campaign's spokeswoman.

But the Human Rights Campaign said the apology "failed to adequately address the real harm and magnitude of his words."

"The diversity of our movement is our greatest strength, and Senator Kirk’s remarks were an affront to our most fundamental values. We have therefore voted to endorse Congresswoman Tammy Duckworth, who has been a strong LGBTQ ally in the House of Representatives, and HRC will contribute the maximum amount to her campaign."

Duckworth currently leads Kirk by 7 points in the RealClearPolitics polling index.

Updated 10:36 p.m. Sunday