Johnny Isakson

The former Georgia lawmaker rose from the ranks of the state Legislature to become a U.S. senator known as an effective, behind-the-scenes force for compromise.

Loeffler is the CEO of the financial platform Bakkt, as well as co-owner of the WNBA franchise the Atlanta Dream. The president reportedly wanted Georgia Gov. Brian Kemp to appoint someone else.

In a statement, the senator said his Parkinson's disease was progressing. "I will not be able to do the job over the long term in the manner the citizens of Georgia deserve," Isakson said.

The president again complained about the late Arizona senator during an Ohio speech, even criticizing the McCain family for not showing gratitude to the president over the late senator's funeral.