If the cotton boll weevil spread in the 1840s, it would literally eat away the economic foundation for Southern slavery. The value of slaves would drop and compensated emancipation would become affordable, (particularly after the Gold Rush made it possible to pay slave owners in inflated, newly minted gold currency). Since slaves would be freed gradually, the question of what to do with freedmen (allow them to move to cities, repatriate to Liberia, drive away to Mexico, ) would be worked out. Probably the last states to give up slavery would be states like Virginia and North Carolina that grow a great deal of tobacco that would be unaffected by the boll weevil but might be affected around the turn of the 20th Century by boycotts on slave grown cigarettes in European nations. Slavery could conceivably wind up never actually becoming actually illegal, but becoming rarer and rarer, limited to things like penal servitude and possibly the sex trade.
The basis for expansive federal power would not exist. Big business would also be more inhibited, since big business depended and still does, on federal supremacy to preempt state and local regulation and create national markets and policies. It would be harder to suppress populism. The US might wind up a lot more democratic socialist, though without a 14th Amendment, enforcement of the Bill of Rights would vary widely from state to state and states would still be free to censor news media and criminalize slander or libel or limit procedural safeguards on the rights of the accused, Battles for civil liberties would have to be fought out state by state, as would union organizing. Populist grassroots politics would probably be more common, though and there would be nothing like our big banks today.
There would be no federal lands per se, as public lands would devolve to states as soon as a territory becomes a state, as is the case in Canada and Australia. Interestingly, we might have national health care on the Canadian model, since Canada's system is the result of provinces taking the initiative of establishing provincial Medicare in the 1960s. Again, major variation between North and South and much more live and let live.
And probably fewer wars since wars without a congressional declaration of war would be far more difficult. We might see recall of Congresspeople and Senators by initiative, brought in during the early 1900s as part of progressivism.
