In just this election cycle alone, Mark S. Kirk has called Sen. Lindsey Graham a “bro with no ho,” suggested people drive faster through African-American neighborhoods, and said that Americans killed in a terrorist attack “should be laid at the feet of the Democratic caucus.”

And yet, on Thursday night, the Republican senator from Illinois made his most notorious comment yet.

In a debate, Kirk questioned the ethnic heritage of his Democratic opponent, Rep. Tammy Duckworth, after the congresswoman had recounted her family’s history of serving in the U.S. military. Duckworth is the daughter of an American father and a Thai mother.

“My family has served this nation in uniform going back to the Revolution,” said Duckworth, who lost both of her legs during the Iraq War in 2004. “I’m a daughter of the American Revolution. I’ve bled for this nation.”

Kirk responded: “I had forgotten your parents had come all the way from Thailand to serve George Washington.”

A few seconds of silence followed before the moderator moved on to another question.

Critics, not all of them Democrats, pounced quickly. A spokeswoman for the Democratic Senatorial Campaign Committee called the senator’s comment “racist.”

“A struggling political campaign is no excuse for baseless and despicable attacks, and Sen. Kirk owes Congresswoman Duckworth and her family an apology,” said Lara Sisselman, spokeswoman for the DSCC.

Aides to Donald Trump, whose candidacy Kirk withdrew his support from earlier this year, also took aim at the incumbent. 

“The same Mark Kirk that unendorsed his party’s presidential nominee and called him out in paid ads?” tweeted Kellyanne Conway, Trump’s campaign manager. “Gotcha. Good luck.”

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

Kirk’s campaign initially declined to apologize in a statement issued after the debate. But by Friday afternoon, he tweeted a formal apology to the congresswoman. 

“Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family’s service. #ilsen,” Kirk tweeted. 

The comment would seem to end what little chance Kirk, a social moderate but foreign policy hawk and fiscal conservative, had of winning his statewide race in deep-blue Illinois. The first-term senator, a former congressman elected to the Senate in 2010, was already considered by both parties the GOP’s most vulnerable incumbent.

Kirk has struggled to raise money (Duckworth has consistently outraised him) or attract help from Republican outside groups, at least in small part because of the string of gaffes he made to begin the election cycle. 

Such statements have led to speculation, both publicly and privately, that Kirk’s stroke made him more prone to say controversial things.

Kirk suffered a stroke in January 2012: He would say later that the medical emergency left him so scared he gripped a nurse’s hand en route to the hospital because he didn’t want to die alone. 

The Republican wouldn’t appear again in the Senate until exactly a year later, when he triumphantly climbed the Senate steps with the help of Vice President Joseph R. Biden Jr. But his aides and even Kirk himself would say the experience changed him, a point the senator has even tried to make a part of his re-election campaign. 

In their telling, the longtime politician had become more empathetic and confident to take positions he believed in. (Kirk supported gay marriage long before most Republicans.)

But others have questioned Kirk’s capacity to continue to do his job, most notably this month when the Chicago Tribune, in endorsing Duckworth, cited the senator’s health. 

“Our reluctant judgment is that, due to forces beyond his control, Kirk no longer can perform to the fullest the job of a U.S. senator,” the editorial board wrote. “We are unable to endorse him for another six-year term.”

Kirk, in a stinging rebuttal, called the editorial a “sucker punch.”

Experts say it’s impossible to know for sure what role, if any, a stroke plays in any kind of personality changes. And they caution that questioning the mental faculties of someone who has suffered a stroke — in a way that people wouldn’t if someone were getting older or simply suffered a trauma — might amount to discrimination.

“The only thing you can say is these are valid questions that people could ask,” says Victor Urrutia, director of the Comprehensive Stroke Center at Johns Hopkins Hospital, told the National Journal last year. “But there wouldn’t be any way that you could say with any degree of certainty that actually any of these things that people are noticing are due to a stroke. It’s impossible to say that.”