The Radical Republicans disliked McClellan because he believed in honorable warfare, because he refused to brutalize the Southern population, because he opposed a significant expansion of the size and power of the federal government, and because he would not use his troops as cannon fodder in senseless frontal assaults, unlike many other Union generals. They also disliked him because he disagreed with their approach to slavery: he viewed slavery as "a great evil," but he opposed the unconstitutional imposition of immediate and uncompensated emancipation on the South, which was the same view that Lincoln held for nearly his entire adult life, The Radical Republicans' distorted portrayal of McClellan is the one still found in most books on the Civil War.
McClellan won nearly every battle he fought and never suffered a major defeat. He may have ended the war by 1863 if Washington authorities, mainly Radical Republicans, had not constrained his operations and had not withheld supplies and troops from his army.