You would have to understand causation of crime to get this, and that's pointless to attempt, as Republicans do not understand causation. Their policies on crime are based on fear and a gut reaction to punish.
This is most likely more of a vote against Republicans than for Democrats. Repubs are seen as the "tough on crime" group, and their policies are the ones seen as the ones keeping recidivism high and those stuck in the CJS unable to get out.
Policies such as demonizing victimless activities like drugs, keeping prison records going so that one is never finished doing their time, etc. Republicans are seen as pushing for longer sentences and making it harder to get employment and correct the trajectory of one's life on getting out.
Any look at corruption will show it as being a pretty even occurrence between the parties, so if you are trying to imply Republicans are somehow saints, there is plenty evidence you're wrong. Plenty evidence those who have learned their lessons the hard way know better than to side with the right as well, it seems.