The reasons for their waffling are many, but it apparently all boils down to concerns over "stability"--a euphemism for fears that Egyptians will decide themselves who their leaders are going to be rather than being content with yet another dictatorial or authoritarian regime sympathetic to U.S. and Israeli interests. I would like to think that Barack Obama knows better in his heart, so what is his abject cowardice in the face of these historical events based on? Fear of what Sarah Palin might say if the Muslim Brotherhood gets to play a role in the new government? Fear that he might lose his bid for re-election if the right-wing could claim that he "lost" Egypt?
Whatever the case, if the government cracks down on the movement, which they will indeed do if it falters for lack of international support, Obama and Clinton will have blood on their hands. Come to think of it, they already do, since until the pro-democracy movement began they were content to simply continue decades of U.S. support for the Mubarak regime.
PS--Oh, believe it or not, presidents can resign. Remember Richard Nixon?