evidence: President Joe Biden announced White House chief of staff Ron Klain will step down next week and will be replaced by Jeff Zients, the former Obama administration official who ran Biden’s Covid-19 response operation. [...] Biden hailed Klain in a statement Friday morning, saying when he was elected president he "knew" he wanted Klain to be his chief of staff calling him "as tough, smart, determined, and persistent as anyone I have ever met." [...] Klain’s departure comes at a difficult time for Biden, with a special counsel investigating his handling of classified information after his time as vice president and with the administration and the president’s family facing renewed scrutiny by the GOP-controlled US House of Representatives. An official familiar with Klain’s plans said his decision to step down is not related to the investigation underway about classified documents found at Biden’s private office and Delaware residence, with the decision being made before the special counsel was announced.