An American University professor who called Donald Trump’s 2016 shocking White House win and has predicted the last nine winners of the Electoral College believes that Democrats must impeach Trump to take back the White House in 2020.

Allan Lichtman told CNN’s Don Lemon on Wednesday evening that Trump only has three of his reelection keys working against him. He said impeachment would check off a fourth—the “scandal key”—and could also trigger other keys like spurring a serious primary challenger or even a potential third party candidate like Rep. Justin Amash (R-MI).

CNN noted that “Lichtman’s prediction system is based on 13 true/false statements about the party that holds the White House.” According to his system, the incumbent loses if six or more of the below statements turn out to be false:

1. Party Mandate: After the midterm elections, the incumbent party holds more seats in the US House of Representatives than after the previous midterm elections. 2 Contest: There is no serious contest for the incumbent party nomination. 3. Incumbency: The incumbent party candidate is the sitting president. 4. Third party: There is no significant third party or independent campaign. 5. Short-term economy: The economy is not in recession during the election campaign. 6. Long-term economy: Real per capita economic growth during the term equals or exceeds mean growth during the previous two terms. 7. Policy change: The incumbent administration effects major changes in national policy. 8. Social unrest: There is no sustained social unrest during the term. 9. Scandal: The incumbent administration is untainted by major scandal. 10. Foreign/military failure: The incumbent administration suffers no major failure in foreign or military affairs. 11. Foreign/military success: The incumbent administration achieves a major success in foreign or military affairs. 12. Incumbent charisma: The incumbent party candidate is charismatic or a national hero. 13. Challenger charisma: The challenging party candidate is not charismatic or a national hero.

Lichtman said Trump has already lost the “party mandate,” “foreign/military success,” and “incumbent charisma” keys and impeachment, because of the strong economy, may be the only way to put other keys in play.

“Democrats are fundamentally wrong about the politics of impeachment and their prospects for victory in 2020,” he said earlier. “Without impeachment, however, Democratic prospects are grim.”

The presidential historian added that impeachment is right morally, constitutionally, and politically and said it is the only way to hold a president like Trump accountable.

He also said Trump is so good at “deflection, distraction, and obstruction” that Democrats need to begin impeachment proceedings to “focus everything on one committee” and begin the formal process “where the House is pointing a finger” at Trump and “saying ‘you are guilty.’”