So I don't know if it is true or not but I heard that the Louisiana Purchase had a very strong opposition and that it was actually approved by a very slim margin when voted for in congress. I checked wiki (I know its wiki and unreliable but thats why I am asking) and it seems it is true it also said some congressmen argued that the territory belong to spain and not france to as part of the reason the purchase should not go through.
So my question is
Why was there opposition for the purchase to happen?
The Spanish/French ownership question was definitely part of it. More importantly, though, was the constitutional question. Since this was early on in our Constitution's lifetime, we hadn't really banged out a lot of specifics and applications yet. A lot of people really weren't sure whether it was Constitutionally legal or not to do the purchase.
And could it have been possible for it to not be accepted and for the US not to expand westward?
The question is kinda bugging me and any insight would be appreciated.
It really depends on what goes on with Louisiana. I'd guess, in the end, it'd halt westward expansion. Americans wouldn't have the same sort of inertia for manifest destiny, so there'd be a lack of interest. If we see Britain take over large parts (or all) of Louisiana after the Napoleonic Wars, it'd make it that much harder for the US to get that land. A war of conquest against Britain would be out of the question; we'd get curbstomped. The War of 1812 was more or less a pro-British stalemate (aka loss), and that was with the bulk of the British forces engaged in Europe. A purchase, as OTL is unlikely; Britain didn't need the money. America wouldn't have the same sort of influence on boundary disputes, so that's pretty much out of the question. Overall, it'd be very difficult to get its hands on a British Louisiana.
If Louisiana stays French, then it depends on Franco-American relations after the Napoleonic Wars. America was pretty friendly with France at that point OTL. We could see a bankrupt post-Napoleon purchase (or even just one later on in the wars). At this point, expansion west is probably similar to OTL, but at a slower pace. Britain probably gets somewhat better border deals, since it has an extra decade to assert its claim.