I should be clear: more than people who are in the middle/lower classes in terms of the effective tax rate (not just marginal), and given our current situation, it may have to be even more than it is today (along with cuts) or the alternative is that the system could collapse.
That my friend is the central question of domestic politics. Right now 'the line' is at historically low levels, and we're facing massive budget deficits. And fresh from a US election year where everyone agreed the deficit is the most important issue facing us, the first thing the government did was vote to extend those historically low rates.
The rich already contribute more. Where's the line? 90% tax rate? 99.9999%?