Ethan Rafuse made a creditable attempt to defend McClellan in his 2011 biography McClellan's War, and while somebody other than I could better assess him as a tactician, strategist, or battlefield commander, I think Rafuse does manage to debunk some of the McClellan myths. The famous incident where Lincoln called on him and was turned away, for example, was much more likely to have been due to an attack of the malaria McClellan picked up during the Peninsula Campaign, not him being an egotistical snob. His supposed failure to pursue Lee after the Battle of Antietam is a great exaggeration: his army was exhausted, and the reconnaissance in force he sent across into Virginia after Lee was decisively repulsed.

There is a real problem with biography: we like stories of people to run like film scripts. An egotistical, snobbish, well-bred and incompetent McClellan makes a great contrast to a modest, progressive Lincoln or a hard-working, unassuming Grant, and Rafuse does show this is another case where the characters are not quite that simple. Grant was quieter than McClellan but no less ambitious, and also made mistakes that cost a lot of lives. And we have to also avoid a presentist bias: McClellan's Whig politics are now way out of line with what we would feel to be correct, but his compromising attitude towards the South was shared by a lot of Northerners, and would certainly be seen again after the War when the North abandoned the freedmen and Reconstruction.