What was that line?

Yesterday morning I finally got to read the piece in Time Magazine, August 3, 2009 edition about the last days in the White House and the relationship at that time between Bush and Cheney.

According to the piece by Massimo Calabresi and Michael Weisskopf, Cheney was pushing hard daily for Bush to pardon Libby. Even Libby himself met with Fielding to plea his own case but because Libby showed no signs of contrition, Fielding did not feel that Libby should get the pardon and Bush agreed.

There were a couple of lines toward the end of the piece that really caught my eye. They were in regard to Cheney trying everything in his power to convince Bush to pardon Libby. The two lines were: “The Vice President knew there was a line out there that he was getting very close to but couldn’t cross” says a former senior official. ‘The President knew that he needed to help make sure Cheney didn’t cross that line either.’”

What was that line and why were each of them so fearful of it?