Well in fairness the dems appointed a few judges who surprised everyone by not being reliably far leftists. For example .... uh .... umm ... huh. Can't think of one. But I'm sure it happened, right?
Felix Frankfurter (FDR) and Byron White (JFK) are considered the two on the lib side of the scales who turned conservative.
I think FDR wanted to pack the court with additional Justices as a result. Libs (Eric Holder) are once again saying they will add to the Supremes if they win in the future.