Republican presidential candidate Nikki Haley is again calling for "mental competency tests" for ageing US leaders after Senator Mitch McConnell froze up in two separate news conferences.
"The Senate is the most privileged nursing home in the country," Ms Haley told Fox News.
Mr McConnell, 81, stared into space for more than 30 seconds when asked this week if he would run for re-election.
A doctor has medically cleared him to work. He had a similar lapse in July.
Mr McConnell has led the Senate Republicans since 2006 and has built a reputation as a ruthless political tactician who keeps his party rank-and-file in line.
Members of the Kentucky senator's caucus have not publicly questioned his ability to lead, though they have the option to call a meeting to discuss the matter when the Senate returns to Washington next week.
Ms Haley, a former UN Ambassador and South Carolina governor, is the most senior Republican to speak out about Mr McConnell's health concerns.
She told Fox News on Friday: "I mean, Mitch McConnell has done some great things and he deserves credit. But you have to know when to leave."
Ms Haley, 51, has focused her presidential campaign on the argument that America needs a new generation of leaders. The average age for members of the US Senate is 65.
"I think that we do need mental competency tests for anyone over the age of 75, I wouldn't care if they did them over the age of 50," she said.
"It's sad," she told Fox. "No-one should feel good about seeing that any more than we should feel good about seeing [90-year-old California Senator] Dianne Feinstein, any more than we should feel good about a lot of what's happening or seeing Joe Biden's decline."
During Wednesday's press event in Covington, Kentucky, Mr McConnell was unable to answer questions, which had to be repeated by staff.
The senator suffered from a concussion after falling at an event in March, according to staff.
Brian Monahan, the attending physician of the US Congress, said on Thursday he had consulted with Mr McConnell and his neurology team.
Dr Monahan said in a statement on Thursday: "Occasional lightheadedness is not uncommon in concussion recovery and can also be expected as a result of dehydration."
Republican congresswoman Marjorie Taylor Greene wrote in a social media post on Thursday that Mr McConnell was "not fit for office".
"Severe ageing health issues and/or mental health incompetence in our nation's leaders MUST be addressed," Ms Greene said, also mentioning the age of US President Biden, who is 80.
Democratic congressman Dean Phillips called on social media for term limits for members of Congress and the Supreme Court.
President Biden said on Thursday he did not have any concerns about whether Mr McConnell was fit to do his job.
"He's a friend," Mr Biden told reporters in Washington DC, "and I spoke to him today. And you know, he was his old self on the telephone."