Ever since Lex Luthor won the Presidency in 2000, the DC Universe -- which had, prior to that point, often used real-world Presidents -- has taken to using fictional Presidents. This has the same upside for their storytelling as the fictional cities of the DC Universe does, in that it allows the writers to do anything -- kill the President, make her an alien, etc. -- without anyone thinking that it is an overt political gesture and becoming offended.
The Luthor Presidency was relatively short-lived within the main continuity of comics, but cast a long shadow. The idea of Luthor running for President was reflected in the series finale of Smallville, and used as a plot point in one of DC's animated feature films.
Luthor, who never appeared on-camera in the series, was also named as the President in the short-lived DC sitcom Powerless.
Obviously, Lex has done a lot of things over the years in comics, and some of them have stuck more than others. Running for President is hardly a guarantee.
During the season premiere on Sunday, Supergirl saw President Olivia Marsdin (Lynda Carter) revealed as a Durlan -- a shape-shifting alien. This was not a matter of a sinister alien taking the President's place, but rather an alien who had been living in plain sight on Earth for years, who had run for President and been elected. Fans had known for a while that Marsdin was a Durlan, but it did not come into play last season.
Before the season began, Tron star Bruce Boxleitner was cast as the Vice President, with a character description that suggested he would step up when the country needed him most -- something that certainly suggested, given his job description, that he would be taking over as President.
Given the rising anti-alien sentiment on Supergirl and the fact that it would be basically impossible for a shape-shifting alien to prove that she was born in the country even if she was, it seems likely Marsdin will be impeached quickly, with Vice President Baker likely taking over. As one of the country's most famous anti-alien activists, the Marsdin controversy might give Lex a perfect platform from which to run.
Lex is currently in prison, so they would have to explain how he not only got out, but managed to con enough people into voting for him despite being arrested by Superman, who is one of the only aliens in the world who seems generally immune to the anti-alien criticisms.
Worth noting: DC also recently released a new printing of President Lex, collecting the stories where Luthor successfully ran for President along with Luthor: The Unauthorized Biography by James Hudnall, Eduardo Barreto, and Adam Kubert.3comments
The comic, which was published in 1989, became meme fodder after the election of U.S. President Donald Trump, since the cover image was borrowed from Trump's then-recent book The Art of the Deal. While Supergirl might use any potential parallels to criticize the current administration, it's worth mentioning that Hudnall is a conservative and it is very unlikely there was any political meaning hidden in the cover idea; it was likely just seen as an obvious idea since the book was a hugely successful biography of a world-famous rich guy.
Supergirl airs at 8 p.m. ET/PT on Sundays, before Charmed on The CW.