Dems need to impeach Trump if they want to win in 2020 -- according to a professor who predicted 9 presidential elections
President Donald Trump faces more investigations after being cleared of collusion with Russia in the Mueller probe. (AFP / Eric BARADAT)

Allan Lichtman has successfully predicted presidential elections since 1984. Lichtman is a professor at American University and authored "Predicting the Next President: The Keys to the White House 2016."

CNN's Chris Cillizza sat down with Lichtman to see his predictions so far about 2020. Lichtman explained that there are 13 categories he looks for when predicting who will hold the keys to the Oval Office.

"Trump wins again in 2020 unless six of 13 key factors turn against him. I have no final verdict yet because much could change during the next year. 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," Lichtman said.

Cillizza, then explained how Lichtman's test works, "If six or more of the statement are false, the incumbent loses. If less than six are false, the incumbent wins. Simple," he said.

Lichtman added that Democrats might need to impeach Trump to fulfill the "scandal" category on the test.

"Democrats are fundamentally wrong about the politics of impeachment and their prospects for victory in 2020," he said. "An impeachment and subsequent trial would cost the President a crucial fourth key -- the scandal key -- just as it cost Democrats that key in 2000."

Adding, "The indictment and trial would also expose him to dropping another key by encouraging a serious challenge to his re-nomination. Other potential negative keys include the emergence of a charismatic Democratic challenger, a significant third-party challenge, a foreign policy disaster, or an election-year recession. Without impeachment, however, Democratic prospects are grim."