Joe Biden’s chief adviser was incentivised to keep the president in the 2024 White House race by the promise of a $4m (£3m) bonus if he was re-elected, according to reports.

Mike Donilon, Mr Biden’s top political aide, told congressional investigators on Thursday that he was paid $4m for working on the Democrat’s campaign and would have earned a further $4m had the former president won, a source familiar with his testimony told Axios.

Although Mr Donilon’s salary had already been detailed in the book Original Sin, the bonus was not previously made public. It raises questions about Biden advisers having a financial incentive for him to run despite widespread concern over his cognitive decline.

In his closed-door testimony to the Republican-led House Oversight committee, Mr Donilon, 66, defended the former president’s fitness for office despite reports of efforts to conceal Mr Biden’s increasing fragility by members of his inner circle.