A disgraced leader absolutely does negate the cause they are fighting for. By doing unscrupulous things, a person can't expect the general public to keep supporting their causes. Everything related to that individual becomes tainted and frowned upon by the general public because nobody trusts that particular leader or their causes.
A disgraced leader does not negate the cause he or she fights for. If the cause is legitimate enough, it will still stand even if the leader who supported it has made some bad decision. If the cause falls because of this, then it was not worth fighting for in the first place.
Just because a leader is disgraced doesn't mean their cause is invalid.A leader may have so much charisma that people will believe that they are to be followed even when their hearts and minds say to be careful.This example shows that people can be productive in the morning even when they really would rather be doing s somthing productive.
I do not necessarily think that a disgraced leader negates the cause he or she fights for. It can damage the reputation of the cause a bit, but it does not out right disgrace it. People make mistakes. We are all human. Every side to a story will have a person who makes a mistake. That does not disgrace their cause.
Of course there is more integrity attributed to a cause when its leader is found to be honest and moral. However, a leader is just a person who makes mistakes or even does other things badly. The cause should stand on its own merits and the leader should be changed if necessary.