Donald Trump says he had a front-row seat to John F. Kennedy Jr. and Carolyn Bessette's volatile relationship ... remembering how they fought, made up, then made out at Mar-a-Lago.

The President of the United States called into the hit Fox News show "The Five" ... and Dana Perino -- the former White House Press Secretary under George W. Bush -- mentioned people all over the country are watching the Ryan Murphy show 'Love Story' about the pair's relationship.

President Trump says the two knew each other ... and, believe it or not, he says the two actually got along quite well.

The prez says the two got along so well JFK Jr. even put him on the cover of "George" ... the political magazine Kennedy ran from 1995 until his death in 1999. Trump graced the cover in March 2000.

DJT says John and Carolyn went down to Mar-a-Lago to visit with him ... and, he remembers they would "bicker" and have "little fights" -- before making up and "kissing wildly."

If JFK Jr. had lived, President Trump says he thinks he would've ended up in the Oval Office ... because everyone just loved him.

Of course, Kennedy and Carolyn went down in a plane crash off the coast of Martha's Vineyard in 1999. The National Transportation Safety Board determined the cause was pilot error, specifically due to spatial disorientation. Kennedy was 38, Bessette was 33.