For space constraints, I will focus on the political developments and main points, and not write a tome.
As one reader mentioned the Whig Party did die; its last presidential candidate was in 1852, President Pierce. Essentially, the Whig Party ended because it was a national party and the United States became very sectional in the 1830s to the Civil War. It is fair to say the United States became separated by an industrial, anti-slavery North and slave farming and nonindustrial South. Of course, that is overgeneralization but it gives needed basic understanding.
Historians have hammered in recent decades that slavery was not as unprofitable and inefficient as thought. Nevertheless, the North far surpassed the South in population,industry, railroads, and even was close to Dixie's farm output. When an anti-slavery movement developed in the North, the South grew paranoid about it. The region became ready to fight for slavery, while it still by its view had the chance to perserve it.
While other factors stirred the North on slavery, the Dred Scott v. Sandford case was the biggest. The Supreme Court in that pernicious ruling stated that slavery could be barred no where in the United States, including Northern states. That really energized the Republican Party, which was clearly dedicated to stop the SPREAD of slavery. Southerners misunderstood many Republicans, such as Abraham Lincoln. Lincoln did not want slavery to spread out of the South, but was willing to leave it as it was.
Basically Northerners and Southerners came into great conflict. Congressmen of the era in session actually had guns and knives! One strong Northern opponent of slavery was severely injured by a caning from a pro-slavery Southern representative,.
Tensions mounted and possible compromises and efforts at peace failed in the heated atmosphere. The country split. The Democratic Party, which has been a national party, divided into a Southern and Northern faction in 1860. Because of the great population growth the anti-slavery Republican Party won in 1860 with Abraham Lincoln in a very heated, divisive atmosphere. Before the election, the South could have kept the status quo, but the Fireaters stirred the region into secession. Lincoln was in a difficult position in that he would have essentially had to cave into the South to prevent war. That would have destroyed his political credibility with the Republican Party.
The key conclusion, which unfortunately must leave important factors out, is the main happening in America before Lincoln's election that a Civil War had become an inevitable conflict by 1857.
Teaching, taking Civil War classes