Democratic presidential candidate Julian Castro has denied his attack on Joe Biden in a Democratic debate was a veiled jab at his mental faculties.
Three times on Thursday night, Mr Castro accused Mr Biden of “forgetting what you said two minutes ago”.
But following the debate in Houston, Texas, Mr Castro denied to the BBC that he was going after Mr Biden’s age.
A political fact-checker ruled that Mr Castro was actually himself incorrect about Mr Biden’s remarks.
Few candidates have been willing to directly broach the age of 76-year-old Mr Biden, who has made repeated gaffes and flubs on the campaign trail.
During Thursday’s ABC News debate, Mr Biden gave what many viewed to be a rambling answer to a question about race in America, suggesting parents use a “record player” to educate their children.
He also at one point referred to the Vermont Senator Bernie Sanders on stage as “the president”.
Another candidate, former President Barack Obama’s housing secretary, Julian Castro, aggressively sparred with Mr Biden over whether Americans would be automatically enrolled in a government-run healthcare plan.
Mr Castro, 44, suggested that 10 million people could be left uninsured because of Mr Biden’s plan, and that the former vice-president had somehow contradicted himself.
“Are you forgetting what you said two minutes ago?” Mr Castro said. “Are you forgetting already what you said just two minutes ago?”
“I mean, I can’t believe that you said two minutes ago that they had to buy in and now you’re saying they don’t have to buy in.
“You’re forgetting that.”
PolitiFact, a fact-checking website, rated Mr Castro’s claim “mostly false” and said he was misinterpreting Mr Biden’s plan.
On stage, South Bend, Indiana, Mayor Pete Buttigieg scolded Mr Castro, saying: “This reminds everybody of what they cannot stand about Washington.”
Mr Castro retorted: “That’s called an election, Pete.”
Democratic critics said Mr Castro’s remarks were uncivil and harmful to party unity.
Political strategist Paul Begala, who worked for ex-President Bill Clinton’s 1992 campaign, called it a “disgraceful crack” and predicted it would damage Mr Castro’s White House hopes.
Fellow 2020 candidate and Minnesota Senator Amy Klobuchar, 59, later told CNN that the attack was “not cool”.
She said it was “so personal and so unnecessary” and “like something that Donald Trump might tweet”.
After the debate, Mr Castro insisted he was not making any reference to Mr Biden’s mental acuity.
He told the BBC: “Whether its Vice-President Biden or somebody else, if somebody on that debate stage says one thing and then two minutes later they deny that they said it, I’m going call them on it. And that’s what I did.”
Mr Castro took to the airwaves on CNN, CBS and ABC on Friday morning to again deny he had been making a personal attack on Mr Biden.
“I wasn’t taking a shot at his age,” he told ABC. “It’s about the healthcare policy.”
Mr Biden served as a senator for Delaware from 1973-2009 before becoming Mr Obama’s running mate in the 2008 election.
He is not the oldest candidate in the race. Mr Sanders is 78.
The eventual nominee is expected to face President Trump, who is 73.