Julian Castro, the Democratic presidential hopeful, was facing a backlash on Thursday night after calling into question his rival Joe Biden’s memory during a debate.

Mr Castro, the former housing and urban development secretary, repeatedly accused Mr Biden of forgetting details of his healthcare plan in a heated exchange on stage.

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The comments forced the spotlight onto the age of the former US vice president. Mr Biden would be 78 on day one in the White House if he wins the contest – the oldest person ever elected to the US presidency.

Mr Biden’s campaign released a fundraising email on Friday accusing Mr Castro of pulling off a “cheap shot” and a “low blow”. 

A number of other candidates seeking the Democratic Party’s presidential nomination for the 2020 election also criticised Mr Castro’s remarks, saying they were “not cool” and “not right”.

Mr Biden also endured criticism over claiming credit for the achievements of Barack Obama, the former president whom he deputised for eight years, with Mr Castro claiming he was the one really carrying on Mr Obama’s legacy.

The clash over memory between the two men was one of the standout moments of the third Democratic debate in Houston, Texas, which saw the leading 10 candidates square off for the first time.

Here’s video of that Castro-Biden clash over health care — with Castro repeatedly saying “are you forgetting what you said two minutes ago?”

Already heard from several Dems who called it an ugly + disrespectful exchange. pic.twitter.com/AcOAtX808U

— Dan Diamond (@ddiamond) September 13, 2019

Mr Biden, the clear front-runner, has faced questions about whether his age would affect his ability to carry out the job of president after a string of recent verbal slip-ups.

At one point on Thursday evening Mr Castro challenged Mr Biden over his healthcare plan. He said Mr Biden’s proposal would not automatically enroll people who were uninsured, unlike his plan.

When Mr Biden denied that, Mr Castro said: “Are you forgetting what you said two minutes ago?"

He went on: “Are you forgetting what you said two minutes ago? I can’t believe that you said – two minutes ago – that they had to buy in and now you’re forgetting that.”

It was not their only fiery exchange. At another point Mr Castro, who is three decades younger than Mr Biden, claimed he was carrying the flame of the Obama administration, which he also served in.

“I am fulfilling the legacy of Barack Obama and you are not,” Mr Castro told Mr Biden. The latter shot back: "That will be a surprise to him."

After the debate, other candidates who were on stage questioned the tone of the attacks, especially over Mr Biden’s memory. Republicans have questioned the former vice president’s age and suitability for office.

Amy Klobuchar, the Minnesota senator, said Mr Castro’s attack was "not cool” and "so personal and so unnecessary”.

She said: “It feels like something that Donald Trump might tweet." Bernie Sanders, the Vermont senator, said it was “not right” to go after Mr Biden personally.

Beto O’Rourke, the former Texas congressman, also criticised the “personal attack”.

Mr Castro denied he had been too personal.

“I wouldn’t do it differently,“ he told CNN after the debate. “That was not a personal attack, this was about a disagreement over what the vice president said regarding healthcare policy."