I'm not convinced that he is "sinking like a rock." His core support numbers have eroded, but the rest are not at the extremes of past numbers.
It might be more that Obama has found it is impractical or impossible to do what he's promised, and he's lost support for it. In other words, the hope was hype.
I think you've probably hit the nail on the head.
Obama's approval numbers reflect what we already know - there is a big chunk - 40-44% who will never vote for him.
But his real losses, approval-wise have been from minorities and liberals, who are disappointed - but it's not like the GOP will nominate someone who will win those people over - more likely they'll nominate someone who will send those people running back to Obama.
