I used to believe it until I looked at the facts."
No, DiogenesLamp has never read "the facts", he's only ever drank the Lost Cause Kool-Aid.
In fact, expanding slavery was a real live issue in the 1850s over which blood was spilled in Kansas and the Supreme Court weighed in with Dred Scott.
The fact is that many/most Northerners generally hated slavery and wanted to see it gradually abolished in states like Missouri & Maryland, and opposed slavery's expansion into Kansas, Nebraska or New Mexico.
That Southerners wanted to expand is also clear, hence the bloodshed in Kansas, and their failure in Kansas also enhanced the appeals of secessionist Fire Eaters for a new direction.
DiogenesLamp: "Now I recognize it is just propaganda to justify the real motivation for containing the Southern states representation in congress."
Northern Democrats were entirely sympathetic to Southern interests and until 1858 Democrats held majorities in both Houses of Congress, plus the Presidency & Supreme Court.
In 1858 they lost the House, but still dominated other branches.
So slavers were never totally dependent on just their own votes, they always had huge numbers of Northern allies.