evidence: President Joe Biden's chief of staff, Ron Klain, is expected to leave his role in the coming weeks, US media is reporting. [...] Mr Klain has spent spent decades as one of Mr Biden's top aides, first in the Senate and later when Mr Biden was vice-president. [...] Mr Klain is the longest serving first chief of staff of any Democratic president. The role is known to have a high turnover rate - former President Donald Trump, for example, had four chiefs of staff during his four-year term.