Obama was sitting 45% when he ran for re election and won easily...
The 45% number is if he was running against unnamed opposition. But put a name on it and see his past and that number equals up quite quickly especially in this GOP field. There best candidate is a Bush...
Obama had the advantage of being an incumbent. Beating an incumbent requires you to get people who voted for him the first time to say, "Yeah, I screwed up" and voting against him the second time.
Very difficult, which is why only five incumbent presidents have been voted out of office in the last 120 years. of those, one was never elected (Ford) and three others had serious third party challengers that gave their former supporters an out. (Taft, Carter, Bush-43). Only Hoover was so unpopular that a lot of voters could say, "Yeah, that was a terrible idea, let's not do that again."
However, when there isn't an incumbant, it's very easy to give the other party a turn. Okay, you had your 8 years, give someone else a go. Which is exactly what happened in 1952, 1960, 1968, 1976, 1992, 2000 and 2008.
A person who voted for Obama could find himself pretty comfortable not voting for Hillary if she doesn't make a good case. And Bush isn't the best guy to to make that case. He's a terrible candidate. You have to literally forget three wars and three recessions.