In addition, McClellan is responsible for the most costly blunder in US millitary history by not overrunning Lee's forces at Antietam. He withdrew waiting for a moment that would never come. The confederacy lasted three more years and costs 10s of thousands of more Union lives.
It is also worth noting that there was a clamor to fire his Secretrary of War, Edwin M. Stanton. When pressure was exerted to remove the unpopular secretary from office, Lincoln replied, "If you will find another secretary of war like him, I will gladly appoint him." Stanton stayed on until 1868 thru the Johnson administration.