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

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

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During a debate between the Senate candidates on Thursday, Kirk drew criticism for his reply to Duckworth’s remarks about her family history.

Answering a question on Middle East policy, Duckworth — an Iraq War veteran — detailed her family's military service going back to the American Revolution.

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

Duckworth's mother is an immigrant from Thailand, and her father is a World War II veteran.

The Democratic Senatorial Campaign Committee called for Kirk to apologize immediately after the debate.

"Thankful for @TammyforIL's—and her family's—service to this country. It's really not that hard to grasp, @MarkKirk," Clinton wrote.

"The same Mark Kirk that unendorsed his party's presidential nominee and called him out in paid ads? Gotcha. Good luck," Conway wrote.

Kirk withdrew his endorsement of Trump for president in June after the billionaire's inflammatory comments about a federal judge’s Mexican heritage. Earlier this week, Kirk refused to say whom he will vote for on Nov. 8.

Kirk, considered one of the most vulnerable incumbent senators facing reelection, trails Duckworth by 7 points in the latest RealClearPolitics polling average.

Updated at 12:49 p.m.