I figure it this way - no Democrat is going to approve of Trump, and no Republican is going to approve of the Democrats.
So right off the bat, we start with 50/50. 50% approval means that 100% of your own party approved of you - which is a very good rating.
In the case of Trump, he's at 44%, which means about 2x6 = 12% of the Republican party doesn't approve of him. That sounds about right. Never-Trumpers and Neo-Cons put together probably constitute 10-15% of the party, and they don't approve of Trump. The rest of the party does though (2x44 = 88%), which is still very good.
In the case of the Democrats, they have a real problem. Let's use round numbers and be generous and say their approval is 30%. That means 2x30 = 60% of their own party approves of them, and 2x20 = 40% doesn't. That's huge. If 40% of your own party disapproves of you, it's a real problem.
I don't know quite how to figure that one. There aren't 40% Bernie supporters, 40% of the party isn't socialists. Maybe if you put together ALL the super-left types you might hit 40%, but even that would be stretching it. It tells me that there's genuine disaffection among mainstream Democrats, which are the bread and butter of both the voting base and the financial base. It seems to me mainstream Democrats have grown very weary of the focus on "get Trump". There's just too many problems, in every Democrat population center in America (which is most of the big population centers, and even most of the little ones).