What political forces lead to the creation of the Kansas-Nebraska Act? Why did northern Democrats view the Emancipation Proclamation as "unconstitutional and catastrophic"?

Both of these events marked political watersheds for the Democratic Party.
The Kansas-Nebraska Act was the result of a collision between several political forces, all of which were embodied to one degree or another in Illinois senator Stephen Douglas, a Democrat. On one hand, Douglas promoted western expansion, a major political force, by attempting to open the territories of Kansas and Nebraska to settlement. He hoped that a railroad could be constructed that linked Chicago to the West Coast and that it would pass through Kansas and Nebraska. But Southern politicians hoped that such a railroad would originate in the South, and Douglas tried to win their support (or at least not alienate them) by potentially opening the region to slavery by allowing settlers to determine the legality of the institution there. This practice had been established in New Mexico and Utah, which were part of the Mexican Cession, but since Kansas and Nebraska were part of the Louisiana Purchase, Douglas's proposal meant repealing the old Missouri Compromise, which had banned slavery in the territory north of the 36'30 line. The Kansas-Nebraska Act unleashed the debate over the issue of slavery, the most potent force in American politics in the 1860s and one which eventually tore the Democratic Party, and the nation, apart. Northern Democrats embraced popular sovereignty as a solution to the sectional crisis, but Southerners came to believe that any restriction on the expansion of slavery was unconstitutional.
The Emancipation Proclamation was a different issue that challenged the Democratic Party in an even more fundamental way. During the first few years of the war, many Democrats had supported peace initiatives with the Confederacy. They realized that any peace agreement would have to come with the protection of slavery, so they sought to continue the war in such a way as to inflict the least damage possible on the South. General George McClellan, famous for his indecisiveness in battle, was a good example of this line of thinking, which actually influenced his approach to fighting the war. When the Emancipation Proclamation was issued, Lincoln essentially redefined the war in such a way as to make the Democratic approach obsolete. As long as the Proclamation was in effect (it would be unless another President contravened it), slavery would be destroyed by a Union victory. The Emancipation Proclamation was a direct blow to the beliefs of many northern Democrats.
https://www.archives.gov/exhibits/featured-documents/emancipation-proclamation

https://www.ushistory.org/us/31a.asp