McClellan was an able military commander who excelled at organization and led the Army of the Potomac capably. I believe that his Peninsula Campaign was well-conceived and not so well executed. My opinion about his performance at Antietam have been challenged by the argument I read in Harsh's Taken at the Flood.
One of McClellan's flaws, I believe, was that he felt that he needed to take care of every detail himself. It seems he did not trust many of his subordinates, including (or especially) his staff officers, to look after the details. McClellan would look after minor details himself. Part of this perhaps is due to the state of the army when he arrived in Washington in late July 1861, but he continued this even after he had organized and drilled the Army of the Potomac. During the Seven Days, McClellan went personally to scout Harrison's Landing and was not present at more than one battle when the army was fighting important rear guard actions. I think that McClellan was absent from the army while it was fighting for his life may be less of a problem than the fact that he did not designate a commander to control the action on the field -- meaning that each corps commander did as he thought best. This put the entire army at risk and I think this is a fair criticism of McClellan's generalship.
This seems contradictory: First you imply McClellan was a micromanager who didn't trust many of his subordinate officers (which is not true), and then you fault him for letting his corps commanders handle their area of operations as they thought best.
And it's a very good thing that McClellan scouted Harrison's Landing. He was very wise to choose that spot as his new base. His selection of Harrison's Landing forced Lee to send his army back to Richmond. Lee scouted Harrison's Landing and decided that it was too strong of a position to attack, that he would wreck his army if he attacked it.
Another point I would question is why, on June 25 & 26, he immediately began to retreat (or change his base) when the fact was that Richmond was defended only by a very small force.
One, he had already begun moving his base to Harrison's Landing before Lee attacked.
Two, as Lee himself pointed out to Jefferson Davis when Davis faulted McClellan for not moving on Richmond, if McClellan had done so, Lee would have gotten in his rear and cut off his line of communications. Also, the force at Richmond, though small, was very well entrenched.
McClellan might have reinforced Porter north of the Chickahominy or have launched an attack on Richmond. Furthermore, after Malvern Hill, even Fitz John Porter advised McClellan to resume the offensive after Lee's attacks were bloodily repulsed, but instead McClellan resumed his retreat to Harrison's Landing.
You need to read Rafuse and Beatie. McClellan could not reinforce Porter at the time; he tried but could not. The AOP was in no condition to launch an en masse attack on Lee's army after six days of continuous fighting. Lee would have liked nothing better than for McClellan to have attacked him after Malvern Hill.
Finally, I think the true reason for McClellan's failure as a military commander was his involvement in politics.
What "failure"? He won every battle he fought except one, and in that one loss (Gaines Mill) he inflicted ruinous casualties on Lee's army. He was the only general to inflict more casualties on Lee than he suffered when he fought him. He won the critical battles at South Mountain and Antietam, which restored Northern morale, ended Lee's Maryland campaign, enabled Lincoln to issue the preliminary Emancipation Proclamation, and arguably persuaded England not to recognize the Confederacy.
If Lincoln and Stanton had not blunderingly (in Stanton's case maliciously) snatched defeat from the jaws of victory after the Seven Days Battles by forcing McClellan to abandon his advance on Richmond, Lee would have remained pinned at Richmond and McClellan could have advanced under the cover of gunboats along the James River and used his heavy advantage in artillery to inevitably take Richmond. We know from Confederate testimony that they were thrilled when they learned that McClellan had been ordered to send his army north to Pope because they believed that they would be unable to stop his advance on Richmond if he moved along the James River. Lee explained to Davis that moving in this manner, McClellan would advance post by post and that the Confederates would have no way to stop him.
It was obvious that he was a willing tool of the Democratic party.
I think Rafuse and Beatie have refuted this charge.
As long as the Lincoln administration's policy remained conciliatory, there was no real great conflict.
Actually, there was already enormous conflict because of Lincoln and Stanton's disastrous interference with McClellan's operations and Stanton's inexplicable decision to shut down recruiting operations just as the Peninsula Campaign began (which was followed by Stanton's harmful decision to form new units with raw recruits instead of integrating them into existing units).
As Lincoln moved closer and closer toward a hard war policy in the summer and fall of 1862, McClellan became more and more of a liability.
Only if one thought that targeting civilians and violating just about every other norm of civilized warfare was acceptable. I'm not at all sure that Lincoln really agreed with the hard war policy, and the majority of Union generals did not employ the shameful tactics that Sherman, Sheridan, Butler, and Kilpatrick did.
By November of 1862, war aims had evolved and McClellan was an outspoken representative of the opposite point of view. Related to this, I think McClellan's downfall was his association with individuals who were outspoken in their desire to bring the war to a close through negotiation with slavery intact. Whatever his personal views may have been, his association with these individuals made him a political liability for Lincoln.
Again, you need to read Rafuse. Most of this is either wrong or overly general. McClellan believed slavery had to be abolished, and he had little patience with Copperheads/ardent Peace Democrats.
On the other hand, McClellan's argument that the war would be won on the banks of the James River was undoubtedly correct. Lincoln's and Halleck's insistence that the Army of the Potomac fight for the rest of the war on the overland route meant that a decisive battle could not be fought in the east. It was not until Grant returned to the James in 1864 that Lee could be cornered and the decisive battle could be fought.
Bullseye!