Check out the 35th US Congress which served from 1857 to 1859, you'll see that I was exactly correct -- Southern Democrats controlled both houses.
Now check out the 36th US Congress which served from 1859 to 1861, you'll see that I was exactly correct -- Southern Democrats still controlled the US Senate, until they seceded and walked out.
DEMS in 1853, 158 seats or 67%, opposition was 32% Whigs, split on slavery
DEMS in 1859, 82 seats or ~35%, opposition was 55% Republican, united on slavery
Looks like their power was slipping away.
You are quite correct that southerners seceded primarily because they were in the process of losing control of the federal government.
The ironic part, of course, is that their loss of control was precipitated primarily by their desperate attempts to tighten it. The Kansas-Nebraska Act and the Dred Scott decision were attempts to put slavery beyond political discussion. They led directly to the formation of a specifically anti-slavery party, which had not existed before. Up to this point, both parties had included both pro=slavery and anti-slavery groups. For instance, David Wilmost of the Wilmot proviso, was a Democrat. He eventually became a Republican.
In the latter case, southern justices suddenly discovered in the mid-1850s that there was a constitutional right to take slaves anywhere in the country, and reside indefinitely without it having any effect on the slave’s status. This was despite nobody having suspected any such right in the previous 75 years of the Constitution.
This was remarkably similar to pro-abortion justices suddenly discovering in the 1960s that the Constitution held a previously unsuspected right to abortion on demand.
In both cases the Court was attempting to put a political issue out of the realm of politics. Both backfired, making the issue mopre of a political hot button than it had been previously. With minor exceptions, Roe hasn’t led to shooting yet.