Not necessarily in all circumstances, in my view. (And I am not sure what "lose ALL credibility" really means when referring to something as non-specific as "a side" made up of a lot of people who have a wide range of opinions and ways of expressing those opinions.)

Did the 1930s Nazi Party retain any credibility, whilst imprisoning (or worse) their opponents?

These measures were seen at the time, as credible by many (who were not victims of it). Perhaps you would agree that this side did not lose all credibilty? Perhaps you can also explain (or excuse) what you do consider what remained credible - despite the methods used by this side to supress any altenative view?