He has credibility because of a 50 year long career of championing Democratic values such as stopping racist redlining and job discrimination, protecting women rights, saving SS and medicare, cleaning up the environment, beating the NRA by banning machine guns, and actually being able to pass legislation while earning enough trust and having a clean enough record that President Obama chose him to be his VP.
It is a shame that he has been trolled so hard by Trump, the right and the left, that just by inventing shit with edited quotes and removal of context of the situations of the times, he is smeared like this.