To prove defamation a plaintiff must prove:
1) That a defamatory remark (defamatory meaning one that harms ones reputation as determined by a reasonable person) was;
2) published (heard by another);
3) of and concerning the plaintiff; and
4) that the plaintiff was damaged, unless;
5) it was slander per se (defamatory remarks regarding: 1) crimes of moral turpitude; 2) the chastity of a woman; 3) a loathsome disease (leaporsy or VD); or 4) of the plaintiff's occupation or profession) or libel (reduced to permanent form (e.g. newspaper, film, sound recording).
Furthermore the Supreme court has ruled that the Constitution requires a showing of malice when the defamatory remark is aimed at a public official or one in the public eye. To be malicious, under a Constitutional analysis, according to NY Times v. Sullivan, the defamatory remark must:
1) be false; and
2) the defandant knew that the remark was flase or was reckless as to the truth.
Although Moore is annoying, his films probably would not amount to defamation of the president under a Constitutional analysis. Note that libel is not a sperate tort in the US. Isnt the common law system fun?