Paris had been liberated and Japan had been pushed out of the Phillipines, so the Axis powers were pretty much on the ropes; peace seemed to be around the corner, and Hollywood was anticipating decades of movies in which all the villains would all have German accents. We had made staunch allies with the Soviet Union and there was no reason to expect any trouble out of them.
Dewey ran on the same tired accusations of Roosevelt's "wasteful" government spending, although in truth, Roosevelt was responsible for many money-saving efficiencies. For example, they hadn't needed to reprint the White House Stationary for twelve years.
Another campaign issue was Roosevelt's supposed "declining health," which FDR laughed off with his typical wit and verve. He died four months after the election.
Some people looked no deeper than Dewey's appearance: a prominent socialite once called him, "The little man on the wedding cake," but former president Herbert Hoover also noticed Dewey's intellect, saying "No one can grow a moustache like that without having it affect his mind."
His defeat by Roosevelt, however, was only a stepping stone for Thomas Dewey. He would go on to be defeated by Harry Truman.
Franklin Roosevelt: 432
Thomas Dewey: 99