Frodo, do you know what a filibuster is. Democrats have pledged to use a filibuster on any social security bill that: raises the retirement age or introduces any privitisation. That leaves only raising the payroll tax limit as a viable alternative. 55 is not a governing majority when the other 45 don't back the bill.
While the Dems threatening the fillibuster has some impact, it doesn't have as much as you think. Their are enough Republicans (especially Chafee, Snowe, Collins) as well as a few others who have some serious rreservations with the Social Security plan for it to even move foward