You claim that President Lincoln 'barely' won reelection, even though he had 55% of the popular vote, and then claim that as evidence that almost half the people didn't support the war. Franklin Roosevelt won reelection in 1944 with only 53.4% of the popular vote. Does that mean that almost half the people didn't support that war either?
Correct me if I am wrong, but I don't believe that Tom Dewey ever advocated a ceasefire with Hitler as part of his campaign. Thus your analogy is flawed.