The Whig Party really fell apart because it could not reconcile its northern and southern wings, with their respective views on slavery and its expansion. The Democratic party was also divided between north and south on the question of slavery, which led to a partial breakup in 1860. In the presidential election of that year, competing candidates, Douglas representing the north and Breckenridge the south, both stood for election.
Insights into McClellan's personality and his command decisions are surely not reliant on a simple explanation such as his Whiggish upbringing. That was one factor among many, and maybe not even the most important. Rafuse does a good job at delving into McClellan's family background and politics; but as you previously noted, McClellan's overriding concern was in maintaining the Union and the Constitution as it was, and not in conducting a harsh war against civilian resources and property (i.e., slaves). That guiding principle was probably more common than not among Northerners in the early years of the war, so in that regard McClellan was not necessarily out of the mainstream thinking.
Grant and Sheridan were comparable in experience. I don't think Sherman had the same level of experience, but Sherman's obsession with the details of movement was enough to compensate for any lack of experience. One of Confederate General Joe Johnstons problems was that he understood what it took to maintain an army, and many other Confederates were unwilling to address those matters rationally.
Bringing the