America's largest LGBT rights advocate on Saturday revoked its endorsement of Illinois Senator Mark Kirk and handed it to his Democratic rival, Illinois Rep. Tammy Duckworth.

Chad Griffin, president of the Human Rights Campaign (HRC), explained in a letter to supporters that a racially charged comment Kirk made about Duckworth went “beyond the pale for our standards of leadership.”

At a debate on Thursday, Kirk took a swipe at his rival after she talked about her family's military history.

“My family has served this nation in uniform going back to the Revolution. I am a daughter of the American Revolution. I've bled for this nation,” Duckworth said. “Families like mine are the ones that lead first.”

“I've forgotten that your parents came over all the way from Thailand to serve George Washington,” Kirk said.

Duckworth has a Thai mother and an American father who is a World War II veteran and traces his heritage back to the American Revolution, according to Mother Jones.

Kirk apologized in a tweet the following day. “Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family's service,” he messaged, a reference to Duckworth, who lost both her legs serving in the Iraq War.

HRC at first stood by its endorsement, saying that it had endorsed Kirk “based on the strength of his record on LGBTQ equality.” Kirk was the second sitting Republican senator to endorse marriage equality and is a co-sponsor of the Equality Act, a stalled bill that would prohibit discrimination in seven key areas, including employment, housing and public accommodations, on the basis of sexual orientation and gender identity.

Both candidates scored 100 percent on HRC's latest Congressional Scorecard, which measures a lawmaker's support for LGBT rights.

Griffin said the step was a first in the group's 36-year history.

“We are a bipartisan organization and our staff and board make endorsement decisions based on a proven record of LGBTQ equality and a candidate's ability to drive legislative change,” Griffin said. “We endorsed the sitting senator, Mark Kirk, because he has been a strong supporter of our cause time and again, scoring a 100 percent on HRC's most recent Congressional Scorecard. But events this week have gone beyond the pale for our standards of leadership.”

“Attacking someone because of her race and ethnicity is inexcusable for anyone, but especially for a sitting U.S. Senator. 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 has contributed the maximum amount to her campaign,” he added.

According to the latest poll, which was released in early October, Duckworth is leading Kirk by 14 percentage points.