There are exceptions, see Murtha.
Murtha wasn't really a Marine...like Dan Rather and James Carville and he just plays one in front of America's television sets; sort of like George W. Bush played Air Guard pilot in helping an Alabama politician run for Congress when he was supposed to be drilling as one in Texas. (Besides, there are always a few rotten apples that don't get plucked from the tree of freedom the Corps protects--just ask Clayton Lonetree).