Actually, and this is what they teach at legal O course nowadays...if you read Article 88, you will note that it specifically says:
“Any commissioned officer who uses contemptuous words against the President, the Vice President, Congress, the Secretary of Defense, the Secretary of a military department, the Secretary of Transportation, or the Governor or legislature of any State, Territory, Commonwealth, or possession in which he is on duty or present shall be punished as a court-martial may direct.”
So you can trashtalk individual Senators/Congressmen, but can't disparage Congress as a whole...and you can use "contemptuous words" against Governors as long as you're not stationed in their state.
Of course you can always get hit with Article 134...