In his press conference Obama said this:
This is why FDR, when he started Social Security, it only affected widows and orphans. You did not qualify. And yet now it is something that really helps a lot of people. When Medicare was started, it was a small program. It grew. Under the criteria that you just set out, each of those were betrayals of some abstract ideal.
As much as it pains me, Obama is wrong and Krugman is right. Social Security was always a big program. One possibility that I haven’t seen brought up is that Obama confused Social Security with welfare, specifically AFDC (or ADC in 1935) which overwhelmingly went to widows and kids when it started.
It’s not an excuse, but it might be the explanation.