National Review links to a Real Clear Politics article speculating on "The Return of Authoritarian Great Powers," specifically China and maybe Russia. Not for a good long while, I don't think, and the author seems to agree:

But the most important factor remains the United States. For all the criticism leveled against it, the United States -- and its alliance with Europe -- stands as the single most important hope for the future of liberal democracy. Despite its problems and weaknesses, the United States still commands a global position of strength and is likely to retain it even as the authoritarian capitalist powers grow. Not only are its GDP and productivity growth rate the highest in the developed world, but as an immigrant country with about one-fourth the population density of both the European Union and China and one-tenth of that of Japan and India, the United States still has considerable potential to grow -- both economically and in terms of population -- whereas those others are all experiencing aging and, ultimately, shrinking populations. China's economic growth rate is among the highest in the world, and given the country's huge population and still low levels of development, such growth harbors the most radical potential for change in global power relations. But even if China's superior growth rate persists and its GDP surpasses that of the United States by the 2020s, as is often forecast, China will still have just over one-third of the United States' wealth per capita and, hence, considerably less economic and military power. Closing that far more challenging gap with the developed world would take several more decades.

The article includes a vital point about 20th-century history that most European analysts sort of skip over:

Throughout the twentieth century, the United States' power consistently surpassed that of the next two strongest states combined, and this decisively tilted the global balance of power in favor of whichever side Washington was on. If any factor gave the liberal democracies their edge, it was above all the existence of the United States rather than any inherent advantage. In fact, had it not been for the United States, liberal democracy may well have lost the great struggles of the twentieth century. This is a sobering thought that is often overlooked in studies of the spread of democracy in the twentieth century, and it makes the world today appear much more contingent and tenuous than linear theories of development suggest.

Definitely go check it out.