Given FDR's health in 1944, he probably was beatable, but it just would have taken a very special candidate.
But nobody outside his inner circle knew how sick he actually was. He put on a show and people believed it. It was in the middle of a war so he wouldn’t have let anything that could reduce his standing as a leader get out.
Dewey or whoever else could have claimed that FDR wasn’t in great health but he wouldn’t have much to base it on. He hid that he was paralyzed from the waist down for his entire time in office so it shouldn’t have been too hard to hide something a lot less visible like heart disease.
