The (incorrect) assumption is that JFK and other politicians of yesteryear would hold their same positions if they were on the scene today. I take issue with that. Politicians have always been, with rare exception, persons without any core beliefs, taking positions that they believe will further their careers. Some are better than others at hiding it. John Kerry is among the worst, Bill Clinton among the best.
Had JFK come along in the 2000’s, he would have been liberal, just like Ted “the Swimmer” Kennedy.
Just look at the RFK of 1960 and the one of 1968 (before he got shot).