Actually, the Republicans hated FDR so much, they had an amendment passed to make sure that no president ever served more than two terms again. FDR may have been well-loved by those who voted for him, but the venom of those who did not was quite powerful. If we had approval ratings that were computed so that the approval of those who did like FDR was cancelled out by the hatred of those who didn’t like him, his approval rating would probably be just around 50%. And that was for someone who managed to get elected to four terms.
Eisenhower presided over one of the mellowest times in the US. Voter turn-out was low when he was elected, and Republicans tend to get elected when turn-out is low, so indifference was his friend. His rating may have been relatively high, but he benefited from post-war prosperity that wasn’t the result of anything he personally did.
Kennedy, according to my father (my late father-- this was a conversation that took place a long time ago), was actually not all that popular in his lifetime, and was elected by a lot of people who just liked the alternative less. He became popular in legend after he was assassinated. My father believed that if Kennedy had not been assassinated, he would not have been remembered as a particularly good nor effective president, and that had Johnson run after Kennedy’s second full term, probably would have lost, as there would not have been any reflected glory. As it was, though, Johnson may have chosen not to run.
Also, FWIW, there was less scrutiny of the candidates then than there is now. The public was deceived into believing that FDR could walk normally, and knew nothing of Kennedy’s Addison’s disease. Also, the public may have known vaguely that FDR and Kennedy had affairs, but they didn’t fill the tabloids, and people could put them out of their minds, or even, if they wished, dismiss them as rumors. The private lives of the candidates were not in people’s faces, and if some newspaper of radio reporter had tried to make them so, people at the time probably would have reacted badly to the story as being in poor taste, and the reporter would not have been successful with the story. I mean, even in my lifetime, in the 70s, people found prurience and general nosiness in celebrity’s lives as something to be indulged on the sly (and even then, media stars were in a whole different class from politicians). It didn’t really come into its own until the 1980s, and the days of hundreds of cable channels.
So, TL;DR: my answer is that FDR, Eisenhower and Kennedy are not much different from today’s candidates-- save Trump-- it is the public and the way they treat them that is different.
Trump is a bizarre aberration created by the media. I don’t fully understand him myself.
One final note: I think some of the venom Hillary Clinton experiences is simply because she is a woman. I think there are some men (and even some women) who can’t deal with a woman as president. I’d like to forcibly drag them into the 21st century, myself.