Are you sure it is?
And before you reflexively reply, note:
there can be a difference between a law or program being "ruled" to be Constitutional and it actually being Constitutional.EDIT: After hitting submit, I decided to see what I could find. I quickly found the Helvering case.* It has been found by SCOTUS to be Constitutional. BUT, along those lines, peruse this:
Is Social Security Constitutional? by John Attarian
(I am not a huge fan of Lew Rockwell offerings, but that one seems pretty good.)
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*
Helvering v.
Davis, 301 U.S. 619 (1937).