FDR and Bernie Sanders aren't even comparable. President Roosevelt was a formidable politician, not to mention he was relatively young and very charismatic when he was elected President.
I'm not going to bother with speculation about Bernie's potential primary performance because I just fundamentally don't think he's gonna run.
But thinking he's not a formidable politician? You try building a viable left wing third party in a state known most for its moderate Republicans (at the time), win statewide office for 30 years, and then almost beat the entire Democratic party in its own Presidential primary against the only party royalty in the last 100 years not named Kennedy. Come back to me after that and tell me he's not formidable.
