Washington DC - Vice President Kamala Harris said she was ready to serve as leader, in a Wall Street Journal interview published Monday, as voter concerns mount about President Joe Biden's age in an election year.

The 59-year-old Harris has faced growing scrutiny about her own abilities as the first in line to the presidency should Biden be incapacitated or stand down.

"I am ready to serve. There's no question about that," Harris told the newspaper when asked whether voters' concerns about Biden's age meant she must convince them of her credentials.

Everyone who sees her on the job "walks away fully aware of my capacity to lead," said Harris, the first Black, South Asian, and female vice president in US history.

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The interview was carried out two days before a stinging special counsel report portrayed Biden as elderly and forgetful.

Republican Robert Hur's probe into the 81-year-old's handling of classified documents said the president should not face charges, but said he would come across to a jury as a "well-meaning, elderly man with a poor memory."

The White House has hit back hard at the report, noting that Hur interviewed Biden when he was under intense pressure the day after the October 7 Hamas attacks on Israel.

Harris also sprang to Biden's defense, branding the special counsel's report as "politically motivated."

Harris has taken on a growing role in Biden's reelection campaign, focusing on subjects including abortion, ahead of November's vote in which he is expected to face a rematch with former president Donald Trump.

Republicans have frequently targeted Harris and polls show the former California senator suffers from low approval ratings, like Biden himself.