WASHINGTON — The race for the Illinois Senate seat once held by Barack Obama has drawn intense attention as one of the closest in the country. But it also marks a compelling milestone.

Republican Sen. Mark Kirk suffered a massive stroke and is now often wheelchair-bound. His Democratic challenger, Rep. Tammy Duckworth, had her legs blown off while serving in Iraq.

“To have two candidates with disabilities run against each other is uncharted territory,” said Jennifer Duffy, Senate analyst at the nonpartisan Cook Political Report. “What’s in bounds, what’s out of bounds, I think changes.”

And the Illinois Senate race is certainly testing the boundaries. A Republican group quickly deleted a tweet in March accusing Duckworth of “not standing up for veterans.” In Kirk’s case, questions were swirling about whether he had recovered enough to participate in debates.

Kirk, sitting in his Capitol Hill office, a well-worn cane at his side and a wheelchair nearby, didn’t dispute that the stroke had slowed him down. In a 20-minute interview, he spoke slowly and deliberately but traversed topics from Donald Trump to Merrick Garland without a problem. Kirk said he has seen marked improvement since his stroke in 2012 — he regularly climbs stairs and his speech has grown more lucid — although he still faces challenges.

“Last night, my girlfriend had a long dinner to go to, and I was frustrated that I had to, that it took me like a half an hour to walk to the grocery store to go get stuff, and I was thinking this would only be five minutes in the old days,” he recalled, but added that the experience has forced him to be more patient, with himself and others.

Duckworth, who thinks of herself as “differently abled,” has returned to as full a life as is possible after a rocket-propelled grenade burst through the floor of the Blackhawk helicopter she was co-piloting and exploded in her lap in 2004. Sitting in a wheelchair in a conference room at the Democratic Senatorial Campaign Committee — the group trying to win back control of the Senate — Duckworth gushed about her daughter, born two years ago, and said she doesn’t let her injuries slow her down. And she said she doesn’t plan to pull any punches in the campaign.

“I don’t think he would want any allowances made for the fact that he had a stroke, and I certainly don’t want any allowances made for the fact that I don’t have legs,” she said.

Kirk had been in the Senate only a year when he was stricken. He underwent several surgeries and a year of rehabilitation before returning to Washington in a headline-grabbing event in which he climbed the Capitol steps. Duckworth was among those cheering him on at the time.

The Sunday before Election Day, Kirk completed another headline-grabbing stair-climbing feat aimed at sending a message that he’s physically capable to handle another term in office. He walked up 37 floors of the Willis Tower in Chicago. He also released a letter from his physician stating he has made a full cognitive recovery.

His left side still is partially paralyzed, and while he uses a cane on the Senate floor, he is most often wheeled around Capitol Hill by aides. He blurted out a series of gaffes last year that he later walked back, leading some to question if his stroke was to blame.

Kirk called fellow Republican Sen. Lindsey Graham of South Carolina “a bro with no ho,” and he said it was important to help African Americans “so that the black community is not the one we drive faster through." He called the president “Barack Hussein Obama” and claimed Obama "wants... to get nukes to Iran." Those close to Kirk told National Journal at the time that he had always been blunt.

Last month, Kirk made national headlines when he made a biting reference to his challenger’s ethnic heritage during a debate.

At one point in the debate, Rep. Duckworth touted her family’s history of military service. The two-term Democratic lawmaker was born in Bangkok. Her mother is an immigrant of Chinese descent and her father, who served as a U.S. Marine, is of British heritage. She also noted she is a Daughter of the American Revolution through her father's side of the family.

Kirk responded, “I had forgotten that your parents came all the way from Thailand to serve George Washington.”

After an awkward pause, Duckworth responded that she was “proud of both my father’s side and my mother’s side as an immigrant.”

Kirk later apologized for the remarks via Twitter.

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Kirk has said the stroke actually has made him more sensitive.

“To go through the agonizing process of learning how to walk again and write again and speak again makes you much more empathetic to people,” he said.

Kirk said he is running for another term because he believes he will be the “best advocate for the state of Illinois.” He sees himself as one of the more moderate members of Congress — he supports same-sex marriage and was the first Republican senator to meet with Garland, Obama’s Supreme Court nominee.

Kirk was the first federal GOP lawmaker to rescind his endorsement of Trump, when in June he announced he could no longer support the Republican nominee after he questioned whether a federal judge could be impartial, because of his Mexican heritage.

Still, Duckworth and her surrogates have repeatedly branded Kirk as a Trump lackey. President Obama told supporters at a fundraiser for Duckworth and Illinois Democrats this month that Kirk was incapable of being a “check on somebody like the guy running for President at the top of the Republican ticket.”

For his part, Trump on the stump has assessed that Kirk’s campaign is “not doing so well” while dismissively noting “that’s his problem.”

Duckworth, who went to Iraq after her Illinois Army National Guard unit was called up in 2003, was injured the next year and then spent more than a year recuperating at Walter Reed Army Medical Center in Washington.

She now regularly wears a prosthetic in place of her left leg but doesn’t wear one for her right except on special occasions. She said using both could trigger debilitating phantom pain — pain in the limbs that are no longer there. So Duckworth spends most of her time in a wheelchair.

“It affects my life every single day, but that doesn’t mean I can’t live a wonderful productive life, because, again, I have to be worthy of the men who saved me,” she said, her voice catching at the recollection.

Duckworth went on to serve as director of the Illinois Department of Veterans’ Affairs, where she awarded the state's first grants to provide counseling for post-traumatic stress disorder, implemented PTSD and traumatic brain injury screening and helped start a legal assistance program for veterans, among other initiatives.

Obama then appointed her in 2009 as an assistant secretary at the U.S. Department of Veterans’ Affairs, where she spearheaded efforts to reduce veteran homelessness. Since she won election to the House in 2012, Duckworth has helped pass legislation to provide resources to prevent veteran suicide.

She chooses to get health care through the VA rather than using health insurance available to members of Congress.

“I want to show people that the care there is good, and I want veterans to come get the care that they’ve earned,” Duckworth said. “And yes, there will be problems, there’s always going to be problems, but that’s why I’m doing what I’m doing — it’s why I’m running for Senate. We’re just going to have to keep at it forever. And that is a cost of war.”

But Republicans have seized on a seven-year-old court case tentatively set for trial in August in which two employees of an Illinois veterans’ home are alleging that Duckworth retaliated against them for complaining about their boss at the time. The suit had been dismissed twice but an Illinois judge ruled last week that it could go forward.

Kirk’s campaign said after the ruling that her tenure at the state agency was "one plagued by mismanagement and scandal." He told USA TODAY that he’s “the only candidate running for the Senate in Illinois who’s not being hauled into court for misdeeds in the VA.”

Duckworth’s campaign has dismissed it as a “politically-motivated case” and she said Kirk’s attacks are not unexpected.

“As a politician I expect it, that’s the way politics is, unfortunately,” she said. “And I have a thick skin.”

Aamer Madhani, USA Today; Associated Press; and Alexandra Martellaro, KSDK contributed to this report.