Embattled GOP 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 (Ill.) appeared to question his opponent's heritage during a debate Thursday night.

Asked about their long-term plan for American influence in the Middle East, Rep. Tammy Duckworth (D), who is Asian-American, said her family "has served this nation in uniform going back to the Revolution."

"I am a daughter of the American Revolution," she added. "I have bled for this nation."

Kirk — considered the most vulnerable GOP senator up for reelection — replied: "I forgot that your parents came all the way from Thailand to serve George Washington."

Duckworth was born in Bangkok. Her mother is an immigrant and her father is a World War II veteran.

The Democratic congresswoman didn't initially respond to Kirk's remark, but asked by the debate moderator if she wanted to comment, she touted both sides of her family.

"There's been members of my family serving in uniform on my father's side going back to the Revolution," she said. "I'm proud of both my father's side and my mother, who is an immigrant."

He also came under criticism earlier this year when he compared President Obama to a "drug dealer in chief" over a $400 million payment to Iran.

Kirk is facing an uphill battle to keep his Senate seat in the blue state next month. Duckworth is currently leading in the polls by an average of 7 percentage points, according to RealClearPolitics.