Even if Kerry were firmly committed to seeing the Iraq situation through to victory, his position would be like George McClellan's as the Democratic candidate in 1864. McClellan was personally in favor of winning the war, but large elements in his party were for peace (on terms allowing the South to remain independent). Lincoln was convinced that if McClellan won, the war would be lost.
The other resemblance between the two is that McClellan was intensely contemptuous of his commander-in-chief and let everyone know it.
Good comparisons, they fit. Like Bush, Lincoln never let himself get sucked into a war of words with McLellan.