Is America a great power in decline, and if so, when can we expect another nation to overtake us and why?

I think we're a long way from being over taken, but we sure need to get our act together soon or we'll overcome ourselves from within.

It's really hard to say who will replace us when we eventually fall (as we will). It just takes favourable conditions to take a failed state and make it strong, and the USA wasn't so hot under the Articles of Confederation.

As to why, I think it will be from internal factors - chiefly political corruption. Our cycle is unique in that the hegemon doesn't have any hostile, land-based neighbors. There are no barbarians at the gates. Instead, we're shifting from the idea that every man has one vote to the reality that every dollar has one vote. Compound this with the fact that most elected officials seek first to themselves, then to their backers, and to their duties last. How many times have heard officials state they weren't scientists of a particular field, but that the single largest pile of votes gave them the authority to tell actual scientists what was what in their field?

tl;dr: America is becoming a chicken coop where the dog has been slowly replaced by the fox.

Even though everyone likes to talk how China will become the worlds next super power, in all reality its economy is not the greatest, its labor class is aging, and its military especially force projection, and naval combat power is inferior to ours!

I do believe however that our current foreign policy is suffering and we are headed in the wrong direction. Right now we are too inconsistent and seem to be deciding our foreign policy on the fly and reacting to problems as opposed to being proactive.