"The problem with people who say we need a businessman is that the government isn't a business," said historian Richard Norton Smith, who has written several political biographies, including on another businessman president, Herbert Hoover. Smith has also run several presidential museums, libraries and foundations dedicated to Lincoln, Reagan, Eisenhower, Ford and Hoover.

Hoover, a wealthy mine engineer and consultant before becoming president, had significant business experience around the globe. But he's looked upon by many today as a president who didn't meet the greatest challenge of his time, the Great Depression.

And Hoover had far more political experience than Trump, having served as Commerce secretary under two presidents, and run massive relief efforts in Europe during World War I.

"Profit-loss statements don't take into account the irrationality of Kim Jong Un," Smith said of the North Korean leader. "Corporate budgets don't have to allow for military defense. All these kinds of perfectly rational expectations that apply in a corporate world are rarely applicable in the less-than-rational world of politics. ... If you've not been tempered, yourself, in the fires of politics, you're operating at a real disadvantage."