You could argue that it was congress's loss.
Truman won it because he ran, as already pointed out, like hell for reelection, but you make an interesting point here. Truman just barely mentioned Dewey in his campaign speeches, he vigorously attacked the GOP controlled 80th congress, whom he referred to as the “do nothing congress”. We can laugh about this today, considering the fact that this congress passed the Marshall Plan among other things. The congressional sessions since 2011 have been the least productive ever. It was a truly smart move that the president called lawmakers back into session in the summer of 1948 to demonstrate, that they’re unwilling to pass popular measures. Truman used this to attack the GOP, because Dewey was actually a lot more liberal than the mainstream of the congressional Republicans.
