I understand your point about the two parties being very similar but there are critical differences in the voting base and it makes a big difference when it comes to US actions. For instance I think it very likely that Obama approves the Keystone Pipeline as Republicans want. But the Tea Party and many other Republicans want to dramatically cut the social programs and boost defense spending and complete control of Congress makes this much more likely to happen.
I suppose the question in my mind is whether or not one can win an American election without at least paying lip service to climate change? In the last election if memory serves, Romney had to at least pay lip service to it - and I'm unconvinced Obama has done any more.
Taking the international perspective, a lot of people thought things would change when Obama got into power (the first time at least). Yet not only could he not close Guantanamo bay, nor cancel the Keystone XL pipeline - he continued to permit policies such as fracking (not to mention extending the attack on civil liberties, which primarily concerns legislation within the US but still has some knock on effects internationally via things like the NSA spying).
My impression was that it appeared in the end the US pretty much just continued business as usual, with slightly different noises being made?
All that said I guess it remains a possible (and perhaps not unlikely) outcome that the US could go down the same route as Canada, Australia and the UK - where it drifts to the right and attacks science with even more vigour (and here at least Obama pays lip service, vs the direct persecution that seemed possible in the Bush administration). Of course, large vested interests no doubt will pay well to undermine what miniscule increments of progress have been made (I get the impression Obamacare is currently a bigger issue in the minds of republican voters).
Overall, I think the US has lost international credibility with Obama. The policies of the nation that abhor portions of the rest of the world (and excuse those not abhorred for similar practises) are now no longer associated merely with the Bush administration, but more firmly with America itself as a nation.
In light of the above statement I offer some evidence that attempts to execute what I describe are already underway. Rep Paul Ryan the main budget drafter for the Republicans is putting forward his new budget proposal. This budget will drive much of the election discussions and it will have a big impact on what happens.
America already spends a truly insane amount of money on "defence" though? The British empire at least had the sense to go bankrupt fighting two world wars - the US empire seems to me to be well on the way without fighting any really big wars at all...
So it is going to be increases in defense spending and maybe big social program cuts. AGW issues are not going to be paid much attention for some time.
And I daresay an increase in spending on riot police and the apparatus required to maintain and expand a security state? Increasing social inequality still further, particularly in the context of rising food prices and resource stresses will undermine social stability.
One of the positive things about America (and the people living there) is that they seem less docile than some places (the UK is pathetic in this respect in my opinion). I assume you're familiar with the BLM/Bundy dispute, for instance? It's just a pity the willingness to question the government (as technically enshrined in the Constitution - which I think is a real head start compared to most nations) isn't extending to the really important questions of the day...