Some people are considered to have "natural charm." (ie. Jackie Kennedy)
Most politicians' charm is extremely superficial -- it's not a matter of if/when/how they learned to be charming; it's how much they mean their words and actions when intending to be charming.
I had a wise social studies teacher in junior high school, who told us that what a politician says is just a small part of what we should be paying attention to. It's how he/she says it -- observe body language, listen to the tone and timbre of the voice, and many other nonverbal cues to gauge the level of sincerity.
People with superficial charm almost never have congruence between their nonverbal language and their words. It's like the old "look me in the eyes and tell me the truth" bit -- superficially charming people have a problem with that. Unfortunately, most other people haven't learned to tell the difference.