Technically, you have defamed him only if your claim is not true. His actions in office however, bear out whether that claim is true or not. Public officials expect to be talked about and their actions criticized anyway. That goes with the territory. He also has the Secret Service to protect his life from all threats.
A private citizen is another matter, that's why the media is very careful what they say about Sarah Palin these days, and they need to shut up about what they are saying about Rush Limbaugh or they could be charged with defamation. What the media is doing to Rush now is an example (on a large scale) of what personal cyber-bullying is--to the target.
If wisdom's ways you wisely seek,
Five things observe with care;
To whom you speak,
Of whom you speak
And how, and when, and where
And defamation is already illegal! If defamation has occurred, then there is no NEED for more law.
If there is a need for more law, then it must be in an attempt to control something OTHER than defamation.
What, then?