“I have heard, in such a way as to believe it, of your recently saying that both the Army and the Government needed a dictator. Of course, it was not for this, but in spite of it, that I have given you the command. Only those generals who gain successes can set up dictators. What I now ask of you is military success, and I will risk the dictatorship.”-Abraham Lincoln
Lincoln and McClellan had one of the worst relationships an American President and a general have every had. McClellan vehemently disagreed with Lincoln politically, and thought much of the government was corrupt. Lincoln thought that McClellan was incompetent, egotistical, and lacking initiative. There were rumors abound that McClellan might instigate a coup against Lincoln's government for a variety of reasons; the government was too corrupt to continue the war, Lincoln issues an Emancipation proclamation, if Lincoln refused to dismiss General Scott. McClellan didn't even share in his strategic planning with the war department he thought the administration was so corrupt--however, some historians have contended the he did this because his own private war policy was one of negotiated surrender of the South, so that they might keep slavery. This didn't not coincide with Lincoln's war policy. McClellan even spoke of the possibility of a coup with other military officers, and mentioned the possibility to his wife in a letter.
Here's a link with some info on the subject.
So what if McClellan marched the Army of the Potomac on Washington in 1862? Would he have enough support in the military to pull if off? I can't see Lincoln not seeing it coming, so the coup would probably fail to seize the administration, but McClellan could take Washington possibly. Would this cause another Civil War within the Union, set up a dictatorship, or fail miserably?