A fifth column is a group of people who undermine a larger group to which it is expected to be loyal, such as a nation.
en.wikipedia.org/wiki/Fifth_Columnist
In a time when this country is engaged militarily on two fronts and a man goes on Armed Forces Radio and says to the troops that he hope the commander in chief fails, I think he's trying to undermine the troops will to protect the country as well as trying to undermine the president of the United States of America. Somebody had better call Lindsey Graham and ask him why Rush is still on AFN
U.S. Senate Judiciary Committee Holds a Hearing on Wartime Executive Power and the National Security Agency's Surveillance Authority
snip...
GRAHAM: "But here's my challenge to you, Mr. Attorney General. There will come a point in time where the information leads to us believe that citizen A may be involved in a fifth column movement. At that point in time where we will need to know more about citizen A's activity on an ongoing basis, here is where I part.
I think that's where the courts really come in. I would like you and the next attorney general and next president, if you have that serious information that you need to monitor this American's citizen's conduct in the future, that they may be part of a fifth column movement to collaborate with the enemy, I want a check and a balance."
Snip...
"And I would just like the administration to entertain the idea of sitting down with Senator DeWine and others to see if we can find a way at some point in the process of monitoring fifth column movements to have a check and balance system that not only would strengthen the commander in chief's role, it will give guidance to the people fighting the war. You'll have Congress on board. You'll be stronger in courts. And the enemy will be weaker."
http://www.washingtonpost.com/wp-dyn/content/article/2006/02/07/AR2006020700731.html