Asked whether he would forge a relationship with President-elect Trump, Lewis said, "It's going to be very difficult. I don't see this president-elect as a legitimate president."
He added: "I think the Russians participated in helping this man get elected, and they helped destroy the candidacy of Hillary Clinton." Lewis called it a "conspiracy" and added: "That's not right. That's not fair. That's not the open democratic process."
Lewis added that he won't attend Trump's inauguration, which he said is unprecedented in his 30-year congressional career.
Update: Trump hit back at Lewis on Saturday morning, saying Lewis should instead focus on his Atlanta district.
Lewis's comments come from a particularly powerful source: A black member of Congress and major civil-rights figure. While Lewis didn't cite allegations of bigotry and racism made against Trump, the whole thing can't help but hearken back to Trump's own questioning of the legitimacy of his predecessor, Barack Obama. For years, Trump raised questions about whether Obama was born in the United States and thus could serve legitimately as president. Obama eventually produced a birth certificate in 2012, but Trump only acknowledged Obama was born in the United States a few months ago.
Members of the Congressional Black Caucus were particularly incensed by Trump's long-running questioning of the legitimacy of the first black president, saying it amounted to bigotry and a racial dog-whistle. After Trump finally admitted Obama was born in the United States in September 2016, members of the CBC held a press conference to denounce Trump.
At the time, Lewis urged Trump to seek forgiveness.
Lewis's words are sure to reverberate in Washington. The intelligence community has said Russia did indeed attempt to assist Trump in the 2016 election. But there's no real way of knowing whether it was decisive when it comes to putting Trump over the top.
Most prominent Democrats have been reluctant to push the idea that Russia won the race for Trump and directly call into question his legitimacy, though Clinton and President Obama have suggested it made a difference -- if not the difference.
Clinton last month named Russia's hacking alongside FBI Director James Comey's late announcements about her email server investigation as the "unprecedented factors that I don’t think we can ignore" when it came to her loss.
Obama has said he thought Russia had some impact, though he couldn't be sure whether it tipped the scales. "Elections can always turn out differently,” he told NPR. “You never know which factors are going to make a difference. But I have no doubt that it had some impact, just based on the coverage.”
About the only other major political figure prior to Lewis who has outright questioned Trump's legitimacy is former Mexican president Vicente Fox, who tangled with Trump over latter's stated plan to have Mexico pay for his U.S.-Mexico border wall.