Buchanan was of course a Democrat, as was Pierce before him.
If the demoncraps were in control of government then why did the South secede? That makes no sense. Nor is it logical.
Because they thought that Lincoln's election meant that they were losing control of the government.
The Compromise of 1850 was Whig not demoncrap because the South wanted to expand slavery to the West.
It was a bipartisan compromise. Taylor and his successor Fillmore were Whigs. Both houses of Congress were narrowly controlled by Democrats. Henry Clay, a Whig, put the compromise together. Stephen Douglas, a Democrat, got it through Congress. Southern Democrat militants weren't happy with the compromise. Neither were Northern abolitionists.
They had already lost control of the government. Lincoln's election was simply an acknowledgement that they would forever after be second class citizens in the Union.