OTL, the Whig Party collapsed after Franklin Pierce's landslide victory against Winfield Scott and intra-party disputes over slavery. What could have saved the Whig Party?
A few more Upper South Whigs vote against the Kansas-Nebraska bill--enough to defeat it. As Michael F. Holt writes in *The Rose and Fall of the American Whig Party*, pp. 982-3 "Next to Tyler's Texas adventurism...no Whig action did more to destroy the party and bring on the war than southern Whigs' easily avoidable support for the Nebraska Act in 1854, a mistake that many of them later rued." https://books.google.com/books?id=hMkYklGTY1MC&pg=PA982