CHICAGO » Illinois Republican U.S. Sen. Mark Kirk apologized Friday for mocking his Democratic rival’s immigrant background and her claim that her family’s military service dates back to the Revolution — comments that drew wide criticism and threatened an already difficult re-election campaign.

“Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family’s service,” Kirk wrote in a Twitter post.

During a debate Thursday evening, U.S. Rep. Tammy Duckworth said her family has “served this nation in uniform going back to the Revolution.”

Kirk responded that he had forgotten that the congresswoman’s “parents came all the way from Thailand to serve George Washington.”

Duckworth, an Iraq War veteran who lost both legs when the Black Hawk she was piloting was shot down in 2004, was born in Bangkok. Her mother, who is of Chinese descent, was born in Thailand. Duckworth, who graduated from McKinley High School in Honolulu and the University of Hawaii at Manoa, has said her father first went to Southeast Asia while serving with the Marines in Vietnam.

Kirk’s remark was greeted mostly by silence in the auditorium of the University of Illinois in Springfield on Thursday evening. Elsewhere, there were quick calls for him to apologize. The Democratic Senatorial Campaign Committee immediately called the comment “offensive, wrong and racist.”

Duckworth, the first Asian-American congresswoman from Illinois, later tweeted a photo of herself with her parents, including her father in uniform displaying his medals. Her tweet says: “My mom is an immigrant and my dad and his family have served this nation in uniform since the Revolution.”

Kirk left the debate Thursday night without speaking to reporters. Campaign manager Kevin Artl said Thursday the senator has called Duckworth “a war hero in his commercials and he commends her family’s service.”

Kirk’s campaign said the senator tried to contact Duckworth by phone to apologize Friday before posting his apology on Twitter. Duckworth’s campaign confirmed he had reached out, but it was unclear whether the candidates spoke.

Kirk’s comments drew heavy scorn across social media. Democratic presidential nominee Hillary Clinton posted on Twitter that she is thankful for Duckworth’s and her family’s service, adding: “It’s really not that hard to grasp, Mark Kirk.”

The same Mark Kirk that unendorsed his party's presidential nominee and called him out in paid ads? Gotcha. Good luck. https://t.co/IV7miL317s

— Kellyanne Conway (@KellyannePolls) October 28, 2016

Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family's service. #ilsen

— Mark Kirk (@MarkKirk) October 28, 2016

Thankful for @TammyforIL's—and her family's—service to this country. It's really not that hard to grasp, @MarkKirk. https://t.co/rKY0rberiB

— Hillary Clinton (@HillaryClinton) October 28, 2016

Donald Trump’s campaign manager also took the opportunity to jab at Kirk, who earlier this year withdrew his support for the GOP presidential nominee and has been a vocal critic.

“The same Mark Kirk that unendorsed his party’s presidential nominee and called him out in paid ads? Gotcha. Good luck,” Kellyanne Conway posted on Twitter late Thursday.

Kirk, who suffered a stroke in 2012 and returned to work one year later, is seen as one of the Senate’s most vulnerable Republican incumbents. The first-term senator from the Chicago suburb of Highland Park is running in a state that leans heavily Democratic, particularly in presidential election years.

Kirk has worked to distance himself from Trump and the GOP, saying he is an independent voice who can work with Democrats to get things done. He often notes his “F” rating from the National Rifle Association and that he broke with his party to call for hearings on President Barack Obama’s pick for the Supreme Court.

But Kirk has complicated his own re-election bid with his tendency to make off-color statements. In August, he said Obama was acting like the “drug dealer in chief” when the U.S. made a $400 million payment to Iran contingent on the return of U.S. prisoners.

He apologized in 2015 after referring to South Carolina U.S. Sen. Lindsey Graham, who’s unmarried, as a “bro with no ho.”

During his first bid for Senate in 2010, Kirk acknowledged that he had exaggerated some of his own military record, including stating that he came under enemy fire while flying reconnaissance missions in Iraq as a Navy intelligence officer.

Democrats consider Duckworth’s success on Election Day one of the keys to reclaiming a majority in the chamber. The second-term congresswoman from the Chicago suburb of Hoffman Estates has a comfortable lead in the most recent polls, but Kirk says the race is closer than people think.