What is moral authority?

Put the phrase "moral authority" into a Google search, and you will get back something over 670,000 hits.  Clearly the expression gets used a lot.  But what do people mean when they use it?  Many people seem to think that it means the right to weigh in on discussions involving what to do about some tough issue.  Other uses suggest that it is a measure of virtue; those who live exemplary lives have moral authority.  Or, that one can gain moral authority by having been put through a trial: the John McCain effect.  One simple definition is that moral authority is the capacity to convince others of how the world should be.  This distinguishes it from expert or epistemic authority, which could be defined as the capacity to convince others of how the world is. 

What do these simple definitions leave out?