See
http://www.cbo.gov/publication/45471for latest CBO projections.
These projections already takes assumes revenue will rise from 17.6% of GDP to 22%-23% of GDP over the next 50 years. But it seems spending will easily outstrip that.
It seems that revenue has to go up and keep the social programs as comprehensive as possible so it is not seen as a redistribution to the poor (Dem position) or various social programs will have to be cut it back plus means testing which in turn will make it look like a redistribution program which makes it more likely they can be cut (GOP position.)
Since I am working now I am for the GOP position, once I go into semi-retirement and then full retirements over the next few years most likely I will shift to the Dem position as I would not have that much earned income for the government to tax but means testing of social benefits will kill me. I definitely see myself voting Dem by 2020.