One-third isn't much and losing territory isn't palatable. There needs to be something better than a forever war, and also better than losing territory. Russia needs guarantees that NATO troops won't be in Ukraine posing a risk, and Ukraine needs guarantees that Russia won't expand on its expense. All of that should also consider the self-determination rights of the Russian minority living in Ukraine as well.
A critical mass is needed for major political changes.
For a referendum that's 50%+1, but for organic political changes I've seen, like when my country legalised abortion or gay marriage or stopped The Troubles, you need a broad consensus.
It's hard to quantify a broad consensus, but it's certainly not 2 opponents to each proponent.