"While running a successful hotel business is honorable and hard work, there is no comparison to running a sophisticated global operation such as the U.S. government," Akerson wrote in an Op-Ed Thursday in the Washington Post. "Trump is simply not up to a job of this complexity."

Akerson was most recently the CEO of General Motors (GM), taking over the top job a year after its bankruptcy and federal bailout. He oversaw its return to profitability and its initial public offering. He retired in January 2014. Before that he was a telecom executive who served as CEO of XO Communications, Nextel Communications and General Instrument Corp.