It's because they have been raiding the Social Security fund for years to give tax cuts to the rich, subsidies to the corporations, more bloated contracts for military toys, and to fund unnecessary wars. Now they don't want to pay that money back, and if they cut benefits and raise the retirement age they won't have to. The Republicans want to use money workers have paid in to the Social Security program to pay for their own proliferate spending.
Don't let the Republicans (and blue dogs) steal retirement money from hard-working Americans! It's the big-spending Republicans' giveaways to the rich that must be cut -- not Social Security!
And for those of you who still doubt that it's the Republicans who are the biggest spenders, view the following graphic: