A Republican senator has been widely criticised for his attempt to undermine his female opponent - a disabled army veteran - by highlighting her ethnic heritage.
During a debate for the Illinois senate, Democratic representative Tammy Duckworth said that: “My family has served this nation in uniform, going back to the Revolution. I’m a daughter of the American Revolution.”
Mr Kirk was given 30 seconds to respond to the same question.
He turned to her and said: “I had forgotten that your parents came all the way from Thailand to serve George Washington.”
Two-term lawmaker Ms Duckworth, after an awkward pause, responded that she was proud of her parents' immigrant status.
She later tweeted: "My mum is an immigrant and my dad and his family have served this nation in uniform since the Revolution."
The representative was born in Bangkok. Her mother is of Chinese heritage and her father is of British descent.
As reported by the Washington Post, Eleni Demertzis, a spokeswoman for his campaign, said that he has called her a “war hero” and honored her family’s service to the US.
“But that’s not what this debate was about," she said.
"Rep Duckworth lied about her legal troubles, was unable to defend her failures at the VA [Veterans Affairs] and then falsely attacked Senator Kirk over his record on supporting gay rights.”
Mr Kirk then backtracked on his refusal to apologise and tweeted later on Friday: "Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family's service. #ilsen."
Hillary Clinton jumped on the spat, thanking Ms Duckworth and her family’s service to the US military.
“It’s really not that hard to grasp, Mark,” she added.
Mr Kirk withdrew his endorsement of Donald Trump in the summer after Mr Trump expressed doubt that an Indiana judge could treat him fairly as he was of Mexican heritage.
Mr Trump's campaign manager, Kellyanne Conway, also seized on the debate to denounce Mr Kirk.
"The same Mark Kirk that unendorsed his party's presidential nominee and called him out in paid ads? Gotcha. Good luck," she wrote.
Mr Kirk suffered a major stroke in 2012 and is vulnerable to lose the election, with a recent poll from independent firm Victory Research finding that 53 per cent of voters did not believe he could carry out his duties.
The Chicago Tribune decided to endorse Ms Duckworth, an army veteran who lost both her legs during the Iraq War, over Mr Kirk because they believed his health would impair his professional abilities.
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