Well he's essentially destroyed the Republican Party in presidential elections. They're still able to perform well in House elections thanks to gerrymandering (2010 was an extremely unfortunate year for them to perform well), but Democrats have built a solid coalition. The states where Obama outperformed his national margin add up to 285 electoral votes.
Ohio and Florida don't coronate the president anymore. The fact that Democrats still win them is a bonus, as well as red states like North Carolina and Georgia (2016, baby).
If Democrats do lose the Senate in November (which I'm skeptical of, as you're aware), it doesn't matter since they'll win it right back in 2016. Probably with a bigger margin than they have now.
The biggest problem the GOP has right now is that they don't have anybody who is actually seen to fix problems. Whatever you may think of them as solutions, Medicare Part D and No Child Left Behind were solutions to problems.
Now, the Republican Party doesn't even do that and they only thing they offer is the destruction of things people actually like - ie. Medicare and Social Security.
A Republican who made the case that Obamacare is bad, but we've got a better plan that will help people, we can streamline government by cutting these specific programs, and so on, but we still need to protect x, y, and z could still win an election.
But, the base doesn't want that anymore. So, here's what the GOP is left with for 2016.
A Blue State governor who has Red State positions on social issues and is an asshole about his other positions. Good luck against the first woman Presidential nominee when you sound like every woman's asshole ex-boyfriend or ex-husband they've had.
A guy who made his bones destroying unions, but has ethical problems and never graduated college.
A loon who makes Peter King seem reasonable.
A couple of retread Governor's who couldn't win the nomination against a flip flopper and a whole bunch of scam artists. Or who failed in their first step on the national stage.
And a guy who could be dangerous if he hadn't had a disastrous response to the SOTU and got kneecapped by his own party on his pet issue.