This is enough to cost him the election in November. It's possible now.
His Presidential campaign will never get off the ground now. The big donors will not see him as a winner. And, again, that's assuming he's re-elected.
It was a comment more dumb than Gerald Ford saying, "There is no Soviet domination of Eastern Europe." during the second debate with Carter.
Thankfully, it's 2018, and no "gaffe," real or perceived, is ever going to cost someone an election again.
And for the record, I agree. America has an above average, but still deeply flawed, problematic, and misunderstood, history.