From the article Bowser linked:

Bush's pledge to make permanent his tax cuts, which are set to expire at the end of 2010 or before, would reduce government revenue by about $1 trillion over 10 years, according to administration estimates. His proposed changes in Social Security to allow younger workers to invest part of their payroll taxes in stocks and bonds could cost the government $2 trillion over the coming decade, according to the calculations of independent domestic policy experts.

In other words, due to the strange view Democrats have that all wealth is by nature the property of the government, leaving $3 trillion in hands of the private sector economy is a "cost." Having retirement funds invested in productive wealth-generating activity instead of simply being wealth-transferred from one citizen to another is apparently a bad thing.

The last thing on Earth the Democrats want is to have the yoke of Social Security lifted from the backs of the elderly and those who support them. If more people start relying on private retirement solutions, where's the political power to be had in that?