In the last episode of season 2, President Walker goes to Camp David amidst the looming fate of impeachment. By then, Frank Underwood and him already have a fallout, and the President refuses to speak to Frank directly.
Frank writes a "heart-felt" letter to the President, and the President immediately changes his mind on Frank, and that proves to be the final nail in the coffin. How can a President of the United States of America, the most powerful man in the free world, the one who rose to the top despite the treacherous DC politics, the one who just had an epiphany about who Frank really is one episode ago-- Frank is just manipulating everyone including the president to serve his own lust for power, can change his mind so suddenly? It seems to me President Walker is so gullible that he definitely deserves to fall.
Really, I can't believe that any barely competent politician can be this gullible, let alone a battle-harden President. Any explanation on why this President is so easily being manipulated?
Well, I know he has to be this gullible for Frank to make it to the top in the shortest time possible, but the whole plot is simply unrealistic.