In an exclusive interview with Express.co.uk, the author characterised Mr Trump's politics as tending towards fascism.
He also argued that the Republican Party had to turn its back on Donald Trump and "Trumpism" if it was to have a political future.
Mr Unger said: "I have always been wary of attributing terms, but yes I think his work is fascism in terms that it is authoritarian, racist and anti-democratic.
"There are many ways to define fascism. What he was doing was dictatorial and he was using every loophole in our government to dismantle the checks and balances that assure at least some fairness."