Still plugging away at Ken Burns's The Roosevelts. Doris Kearns Goodwin pushed the idea
On a political level, polio might have made him feel greater genuine empathy and sympathy for the problems of common people. However, he was very much a progressive before polio deprived him the use of his legs. As a politician, he craved popularity and the New Deal was nothing if not very popular with voters, as is clearly evident in his four elections. If anything moved FDR left it was public opinion and the desire to win votes. Social Security is a great case in point. FDR wanted to draw the support Dr. Townsend was building around his proposal. The Social Security Act that FDR signed was different in many ways from what we now know as Social Security. It covered far fewer people and was setup as a self funding program (demonstrating a fiscal conservatives that lurked in FDR). It is hard for me to see how the New Deal would have been very different, or that FDR would have not adopted old age insurance, if had not been afflicted by polio.