While McClellan was fairly brilliant logistical commander and one of the first to truly understand the dynamics of railway war, he is generally regarded as a very poor battlefield commander. The criticisms are three:

1. Despite maintaining a 2:1 to 3:1 advantage against confederate forces, he refused to engage in open warfare when the early stages of each campaign, when it could be decisive. He refused to consult superiors for both intelligence and strategic matters or trust subordinates with the privilege of initative. This despite that he often over-estimated his own ability to juggle things and was rarely where he was needed, and instead was into political intrigue, such as forcing the resignation of the man who early on defined the architecture the eventual victory - General Scott - and mocking Lincoln.

2. During the Peninsula Campaign, McClellan pulled of a logistical feat in moving his entire army via ship. Unfortunately, that's about the only thing that went right. Yorktown required a siege, which slowed him down at the precise moment he should have been capitalizing on the surprise. Once he was ready for the siege, the confederates fled, and Lee foolishly gave chase, sans heavy artillery which could not be moved quickly.

McClellan's army defeated the confederates at Seven Pines, but then inexplicably did nothing for the next three weeks, while Lee took over, and strengthened the defenses of Richmond. At this point, McClellan started jumping at shadows after being spooked by Stonewall Jackson and then completely fooled by Magruder. In the meantime, his artillery was still hanging around Yorktown. What's worse is that he basically did all of this from a gunboat, and was never in harms way, leading to charges of dereliction of duty while his army really didn't do anything. McClellan retreated back to Washington, but refused to play nice with Pope by giving him the resources needed for Pope's military tasks. Lee judged that McClellan would not attack him, and safely pulled many of his own troops to defeat Pope at Second Bull Run.

During the Maryland campaign McClellan was actually given a intercepted copy of Lee's orders. He was even convinced that the orders were legitimate. Yet still he was excessively cautious and did not exploit the advantage. Lee soon found out the orders were exposed, and took advantage of McClellan's (now incorrect) knowledge of the orders. At the Battle of Antietam, Lee once again out-maneuvered and fought McClellan to a stalemate, despite McClellan having a 2:1 advantage in manpower. McClellan's chief flaws - inability to share information, orders and strategy with subordinates and superiors, and his excessive caution again causes massive issues with McClellan being unable to take advantage of Lee's withdrawal.

tl;dr He was cautious, political, inefficient, could not delegate, and could not win against Confederate Generals, even when presented with every single possible advantage.

He then chose to run for President as a Democrat to kick Lincoln out of office, and was thrashed. After his death, McClellan's personal letters, which where highly inflammatory against Lincoln and the Republicans, were published by his estate. This bought him a loyal following among "the lost cause" historians, but solidified the historical perspective that he was a able organizer, talked a good deal, but never a capable battlefield commander.

Because Lincoln and other Republican military amateurs repeatedly put him in terrible situations, failed to make best use of his talents, changed the kind of war they were fighting, and did not understand what he was doing.

From late 1861 to March 1862, McClellan was General in Chief of Union armies, giving him command authority over all the Union's field forces. He was still commanding general of the Army of the Potomac, but he had much greater vision to carry out in his position as General in Chief. At this stage, McClellan and Lincoln broadly agreed on strategy. The Union had superior numbers, and the South had interior lines; the Southern advantage could trump the north's superiority if those numbers were not pressed forward simultaneously. Union military operations had to be mutually supporting, and cover a broad front. They also believed that the South had been largely hoodwinked into secession, and that most Southerners were moderates and Unionists, who were loyal citizens of the United States until proven otherwise.

McClellan was also a Whig by inclination; conservative, distrustful of ideologues north and south, a believer in compromise. He fought the war to protect the federal-republican form of government from being undone by extremists. The way he saw it, the more blood Union armies shed, the harsher terms the northern public would call for against the defeated south. Unwilling to be reduced to conquered provinces, the south would fight back harder, bringing more blood, and harsher demands, and this cycle would spiral out of control. The Union could never be put back together, unless some dictator took power and destroyed the whole republic, at which point, why bother with the Union anymore?

With these strategic principles in mind, McClellan drew up a plan for a strategic offensive across multiple theatres of the war. The principle army, under his personal command, would take advantage of Union naval supremacy to take the shortest route to the Confederate capital at Richmond, which was the York James peninsula; from there, they could be easily supplied, and once the Confederate capital and their principle army had been invested in a siege (remember, McClellan had reported on the Siege of Sevastopol during the Crimean War), it would be only a matter of time. This is the most famous piece of his plan for spring-summer 1862. However, he also envisioned simultaneous offensives by Grant and Buell towards Corinth->Vicksburg and Chattanooga, respectively. That second objective is particularly salient, as it was largely undefended in 1862, but was absolutely critical to the use of the South's interior lines, one of its principle advantages as Lincoln and McClellan saw.

So McClellan has all these mutually supporting offensives planned, but won't actually take the plunge until they're all ready; Halleck and Buell are in control of the West, and keep telling McClellan they can't move, so McClellan can't move. Grant is the one who ultimately gets the ball really rolling in Feb 1862, when he takes Forts Henry and Donnelson, which guard the Tennessee and Cumberland rivers, respectively. He only did this on his third request for permission from Halleck, who granted it only upon receiving a false intelligence report claiming PGT Beauregard was coming west with 35,000 men. Nashville falls without a fight (Albert Sidney Johnston's orders to fortify the town had been ignored by the governor), and a riverboat raid down the Tennessee river penetrated into northern Alabama. During that time, the riverboats didn't see a single enemy soldier. The Confederate defensive perimeter has been busted wide open. By March, McClellan is ready to depart for Fort Monroe on the Peninsula. All three armies are on track for their objectives.

The problem comes when Lincoln decides to remove McClellan as General in Chief. He's still commander of the Army of the Potomac, but now he no longer has authority over the other armies. No one is keeping them all on track at the top, and Henry Halleck adheres too slavishly to Jomini, concentrating both western armies and advancing with agonizing slowness on Corinth, while leaving the vulnerable prize of Chattanooga, from which the Confederates will launch their 1862 invasion of Kentucky, unmolested. Furthermore, Lincoln overruled and undermined McClellan prior to departure, and appointed corps commanders for him. Earlier, his division commanders had voted on whether they approved of the Peninsula concept 8-4 in its favor. The corps commanders Lincoln picked had voted no.

So things don't pan out in the west like McClellan hoped, but he still gets his army to the Peninsula fine. However, when they get there, it turns out their maps were wrong. They initially thought the river south of Yorktown flowed West-East, making the town easy to bypass, but since it flowed North-South, it made the town into a chokepoint that could be fortified. McClellan wanted to conduct an amphibious operation on the York River on the north face of the Peninsula and land Irving McDowell's force behind the Confederate fortifications at Yorktown. The Navy, however, refused to cooperate until Confederate guns at the mouth of the river were silenced; McDowell was to proceed towards them over land. However, McDowell's force was removed from his command, so he resorted to a siege of Yorktown.

It was little more than a speedbump to McClellan, though. The Confederates had no way to win at Yorktown. McClellan had dug thorough siege trenches, so they couldn't attack. They couldn't just stand on the defensive, since then McClellan could haul up the siege artillery and blast them to Kingdom Come, and if they retreated, they surrendered Norfolk and the CSS Virginia, the only real hope for a Confederate navy. This is what commanders dream of: a Win-Win-Win scenario with negligible casualties.

The Peninsula campaign proceeds right along, slowly but surely, and there's really nothing the Confederates could do to stop him has he worked his way towards their capital and key industrial center. McClellan had been wanting to base his operations from the James River for some time, and this was a great fear for his Confederate counterparts. Union gunboats could get within three miles of the Confederate capital using the James; its prospects for supplying a siege seemed bright.

However, McDowell's force was finally being transferred to McClellan's command, but not by sea like he wanted; McDowell was marching south over land from Fredericksburg, because Lincoln and Stanton were paranoid about Confederate forces approaching Washinton. To recieve him, McClellan had to extend his right flank far above the Chickahominy River, leaving it vulnerable, and use White House at the terminus of the Richmond and York River railroad as his base. To compound the unfavorable situation, McDowell was redirected to the Shenandoah to chase Stonewall Jackson around, to no avail, and under protest. Lee pounced on McClellan's exposed flank, but McClellan was able to shift his base of operations back south of the Chickahominy River to the James during the ensuing Seven Days Battles. This was an eminently sensible and strategically sound move, but it looked like retreat to ignorant Republican journalists, who leapt at the chance to undercut him.

In addition to his legendarily abrasive personality and party alignment, the political interference McClellan encountered on his Peninsula Campaign and his inability to keep all the armies under his control and on track made it impossible for him to win the war in 1862. Absent Lincoln and Stanton's meddling, I don't doubt the South could have been crushed in 1862.