Mr Klain has served as a top aide to Mr Biden since the 1980s in the Senate and later when he was vice-president.

The ultimate Washington insider, Mr Klain was also a senior White House aide to Barack Obama and chief of staff to Vice-President Al Gore.

He was played by actor Kevin Spacey in the movie Recount, about the presidential election of 2000.

The White House chief of staff - who manages the president's daily schedule and is often described as his gatekeeper - is a political appointee who does not require confirmation by the Senate.

He served as chief counsel on the Senate Judiciary Committee from 1989-92 when Mr Biden was chairman. He was also an adviser and speechwriter on Mr Biden's unsuccessful 1988 and 2008 White House campaigns.

Mr Klain served as chief of staff to Mr Biden from 2009 to 2011, the first few years of Mr Biden's tenure as vice-president in the Obama White House.

He later served as President Obama's "Ebola czar" during a minor outbreak of the deadly disease in 2014.

The longstanding Democratic operative was also involved in both of Bill Clinton's presidential campaigns and was an adviser to John Kerry's failed White House run in 2004.

He has been a presidential debate coach for Mr Clinton, Mr Gore, Mr Kerry, Mr Obama, Hillary Clinton and Mr Biden.