Weaken Slaveholding interest pre 1850

In OTL Slaveholders and their supporters seem to have had a huge influence over the Federal Government. It seems to me that having been used to getting their way was a factor in the crazyness which led to Sessession.

Could that influence have been weakened with a POD of say 1800 or later?
 
1. have the cotton gin not discovered.
2. have some earlier kind of human say Homo Erectus discovered making it morally harder to keep humans as slaves
3. change the rules so that any runaway that was not caught for a year was free.
 

OS fan

Banned
Earlier industrialisation. As long as there was no way to replace slave work by machine work (and no need for former slaves as factory workers), slavery will be able to continue as a necessary evil.
 
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