Decorum in the House and in Committees
Under clause 1(a)(1) of Rule XI ... Members should comport themselves with the rules of decorum and debate in the House and in Committees specifically with regard to references to the President of the United States as stated in Section 370 of the House Rules and Manual.
As stated in Cannon’s Precedents, on January 27, 1909, the House adopted a report in response to improper references in debate to the President. That report read in part as follows:
"It is... the duty of the House to require its Members in speech or debate to preserve that proper restraint which will permit the House to conduct its business in an orderly manner and without unnecessarily and unduly exciting animosity among its Members or antagonism from those other branches of the Government with which the House is correlated."
...
Under section 370 of the House Rules and Manual it has been held that a Member could:
* refer to the government as "something hated, something oppressive."
* refer to the President as "using legislative or judicial pork."
* refer to a Presidential message as a "disgrace to the country."
* refer to unnamed officials as "our half-baked nitwits handling foreign affairs."
Likewise, it has been held that a member could not:
* call the President a "liar."
* call the President a "hypocrite."
...
Section 370 of Jefferson’s Manual states that the rule in Parliament prohibiting Members from "speak{ing} irreverently or seditiously against the King" has been interpreted to prohibit personal references against the President. In addition, Speakers of the House have consistently reiterated, and the House has voted, to support the proposition that it is not in order in debate to engage in personalities toward the President.
http://www.rules.house.gov/POP/house_comm_dec.htm