Don Lemon says Taylor Swift could put Donald Trump through a very cruel summer ... 'cause, if she wants to, he thinks she could swing the election in President Biden's favor.

We caught up with the former CNN anchor Friday at LAX where he warned MAGA supporters to keep an eye on just how involved the singer-songwriter opts to be during this presidential campaign.

Lemon says TS wields a ton of power and influence, and while she's not exactly his generation ... he gives her all the props for fighting for her catalog and becoming a self-made billionaire.

As we reported ... members of Trump's inner circle are reportedly ready to go to "holy war" with Taylor if she decides to endorse President Biden -- as she did in 2020 -- but Don thinks it will take more than one-liners to combat her impact on the election.

Translation: underestimating her could sink DT's White House dreams.

This all falls in line with what FOX11 political reporter/anchor Elex Michaelson said on "TMZ Live" earlier this week ... warning Trump's going toe-to-toe with Swift, and Swifties, is a risky strategy.

Although, he does see a scenario where Swift ends up helping both candidates -- basically, Trump could use a Taylor endorsement for Biden to blast Hollywood elitism, and mobilize his supporters to get out and vote.

There has been a vocal minority hating on T-Swift ... even celebs like Plies, who are getting tired of seeing her all over their TVs when they're watching a football game -- so it's not far-fetched to imagine that hate motivating Trump backers.

We know Biden and Trump have bad blood -- to borrow from Taylor's catalog -- and, while she's yet to make an endorsement, it's likely the self-proclaimed anti-hero will end up impacting the election ... one way or the other.