All the what-if's regards to Dewey becoming President having him winning in 1948 in his second attempt at the Presidency. It is often forgoten that he ran in 1944, when he went down to a 432 to 99 defeat in the electoral college, but that doesn't show that the race was acutally quite close espically in the large states. Dewey lost Michigan (19 votes), and New Jersey (16 votes) by just over 1%, and went down to very narrow defeats in Pennsyvania (35 votes), Missouri (15 votes), Illinois (28 votes), Idaho (4 votes), Maryland (8 votes) and New York (47 votes) (by just 5.01 %) in his home state.
Dewey wins these eight states, he wins the election by 271 to 260 elec votes.
How does Dewey deal with the end of the war, and the Soviet Union in Europe?