Professor Allan Lichtman, an American historian who correctly predicted the previous nine presidential elections, says President Donald Trump is heading to almost certain victory in 2020 — and there is only one possible roadblock to his second term.

What did Lichtman say?

The American University professor, who has correctly predicted every presidential election since 1984, predicted last week that Trump will win re-election.

Lichtman said on CNN that Trump's only re-election roadblock will come if "Democrats grow a spine and do their Constitutional duty, and move into an impeachment inquiry.

"It's a false dichotomy to say Democrats have a choice between doing what is right and what is constitutional and what is politically right," Lichtman explained. "Impeachment is also politically right."

Lichtman went on to debunk the idea that impeachment would backfire against Democrats, explaining the road to George W. Bush's presidency — a major Republican victory — was paved by Republicans pursuing impeachment against Bill Clinton.

What do others say?

Politico reported last month that multiple election prediction models and election experts believe that if the election were held today, Trump would likely win in a "landslide" victory. The experts and their models analyze the economy and other factors — not political polls.

Yale economist Ray Fair, a leading expert in modern political predictions, believes Trump will win in 2020 by a "non-trivial margin."

"Even if you have a mediocre but not great economy — and that's more or less consensus for between now and the election — that has a Trump victory and by a not-trivial margin," he told Politico.

Donald Luskin, who works at a research firm that correctly predicted Trump's 2016 victory, said the strength of the economy is too much for Democrats to overcome.

"The economy is just so damn strong right now and by all historic precedent, the incumbent should run away with it," he told Politico. "I just don't see how the blue wall could resist all that."