"This is all like an alternative universe that he created," Bush said. "The reality is, that's not going to happen. And people are going to be deeply frustrated, and the divides will grow in our country."
Trump and Bush often butted heads after both men announced their intentions to seek the GOP presidential nomination last summer. On the campaign trail, Trump often called Bush "low energy" while Bush labeled Trump a "bully."
Bush suspended his campaign in February after placing fourth in the South Carolina primary.
"I can't vote for Hillary Clinton and I can't vote for Donald Trump," Bush said. "This is the first time in my adult life (that) I'm confronted with this dilemma."
Trump did not respond to Bush's comments.