If we weren't watching Republicans challenge what looks like an inadequate, jerry-built new health insurance reform law in the Supreme Court, we wouldn't be able to imagine how much the shape of the 1935 law was set by trying to avoid having the plan declared unconstitutional.
The result was intrinsically regressive: for the people who could least afford it, those with the lowest wages, Social Security taxes amounted to the highest percentage bite out of their take home pay. That discrepancy lingers today in the cap on the amount of income taxed for Social Security; people earning over about $108000 annually stop paying into the fund at that amount. The rest of us pay on every dollar of wages. Roosevelt understood he was creating a less than fair system.
The political history of Social Security is instructive and fascinating. The issue of maintaining our legal, moral and political right to old age assistance from the federal government is still alive today as Republicans repeatedly float plans to hand our security over the Wall Street.