Truman did have an approval rating of 40% in June '48 and won relection. Dubya had it at 48% right before election night. The favorability rating below 50 is garbage, especially if people are just upset with reality. edit: Just like Obamacare in 2018, abortion will crush the R's in all elections.
Overall first term average was 62%. When Dubya was at 48% he still had more approvers than disapprovers for a net positive approval.The poll a week before and after had him over 50
Also note Trumans disapproval in June 48 was 44%,so he was only upside down by 4 points.Bidens disapproval is 59% and he is upside down by 22 points.
Republicans won in 2010 while Obama won in 2012.Off year elections do not predict presidential elections.