Tuesday night's debate between Pennsylvania US Senate candidates John Fetterman and Mehmet Oz complicated Democrats' hopes of keeping control of the upper chamber after the midterm elections next month.
Political commentators noted Mr Fetterman, the state's Democratic lieutenant governor, at times struggled to string sentences together in his debate against Mr Oz, the Republican TV star, highlighting the lingering effects of a stroke he suffered in May.
Mr Oz, backed by the endorsement of former president Donald Trump, has repeatedly questioned Mr Fetterman's fitness to serve in the Senate.
During the debate, Mr Fetterman used closed captioning technology to assist him during the debate.
Mr Fetterman used his opening statement to address “the elephant in the room”.
“I had a stroke. He’s never let me forget that,” he said.
“And I might miss some words during this debate, mush two words together, but it knocked me down and I’m going to keep coming back up.”
Mr Fetterman released a doctor's note last week saying he was recovering well from the stroke he suffered this year, AP reported.
He at times struggled with his words while trying to attack Mr Oz over the surgeon's stance on abortion.
During the debate, the Republican said an abortion is to be decided by “a woman, her doctor and local political leaders”.
Mr Oz's campaign on Wednesday defended his position on abortion and did not comment on Mr Fetterman's health, Reuters reported.
It was the only debate between the two in the competitive battleground state. Democrats see Pennsylvania and the Georgia US Senate race as key to controlling the upper chamber next year.
Mr Oz has been surging in polls in recent weeks, a reflection of Republicans' growing confidence that they can take control of both houses of the US legislature.
The conservatives were already well positioned to retake the House of Representatives, but now find themselves in a competitive race for the Senate.
Mr Fetterman's advantage over his rival has fallen to 2.3 points, down from a September high of 10.7, data compiled by FiveThirtyEight shows.
President Joe Biden visited him in Pennsylvania last week to boost his political prospects.
Mr Biden and former president Barack Obama are expected to again promote the Senate hopeful next month.
Agencies contributed to this report