The senator is talking about how the abolition of slavery will break the back of the cotton trade.
Step-by-step explanation:
This statement is one of surety of the trade of cotton being lucrative enough for it to allow the overlooking of the race relations and the blatant systematic discrimination it brings upon the population.
The direct statement here is that Cotton is king and no moral stances can have a word against how powerful and how lucrative the trade is for the USA. The abolition of slavery can break the whole trade's profits and hence the senator is firmly on the side of the industry.