A professor who has accurately predicted every presidential election since 1984 says Donald Trump will win reelection if the Democrats do not impeach him.

Allan Lichtman, a history professor at American University in Washington DC, told CNN that he’s confident Mr Trump will win the 2020 election “unless the Democrats grow a spine and do their constitutional duty and move into an impeachment inquiry.”

Mr Lichtman points out that the impeachment trial - which happens publicly - is often forgotten in the current discussion surrounding impeachment.

He thinks that process, in which house prosecutors present evidence, would have a major impact on election.

“Richard Nixon, before all the public revelations, was at 67 per cent approval,” he said, referring to the Nixon's impeachment hearings following the Watergate situation, which led to his impeachment and resignation. “It was the public revelations of his misdeeds that drove him down to 25 per cent.”

Following Robert Mueller’s report, Senator Elizabeth Warren, who’s running for the Democratic nomination to vy against Mr Trump, immediately began calling for impeachment proceedings. She has been followed by several key Democrats.

On Sunday, Justin Amash, a Republican congressman from Michigan, became the first member of the House GOP to call for the president’s impeachment.

Mr Lichtman has accurately predicted the last nine US presidential elections, beginning with Ronald Reagan in 1984. His criteria for predictions lies in 13 key points, including incumbent charisma, challenger charisma, and public arrest.

"Trump wins again in 2020 unless six of 13 key factors turn against him,” he told CNN. “Currently, the President is down only three keys: Republican losses in the midterm elections, the lack of a foreign policy success, and the president's limited appeal to voters."

Mr Lichtman has also been predicting a Trump impeachment since he took office, including saying he thought the president would be impeached by “Spring 2018.” He was not.

“I have no final verdict yet because much could change during the next year,” he said.