“I was not a big fan of the president’s policies, as you know, either domestically or internationally,” he said. “But the results of his mistakes and errors, in my opinion, have been more severe than even I would have predicted.”

Mr. Romney, former Massachusetts governor and 2012 Republican presidential nominee, pointed to unrest in the Ukraine, the rise of the Islamic State terrorist group and stubborn economic woes in the United States.

“I think he underestimated the plans and objectives of [President] Vladimir Putin in Russia. I think he made mistakes in Iraq by not putting in place a status of force agreement. And a number of things where I think he was in the wrong posture,” he said.