Earlier quoted context omitted.
For sure, if you can trust your partner to assist you with this, and if you can guarantee that your partner will not face legal consequences, this would be a great option. Dementia poses a legal wrinkle, in that in many jurisdictions, assisted suicide requires consent at the time of the procedure. In many states in the US, especially in the south, assisted suicide is a boogeyman that runs against prevalent religious…
I'm fairly certain in the US you cannot legally consent to being killed.
There's also an interesting bit of case law and state statutes that deals with "drug cocktails", which when combined, will lead to death. A physician can prescribe these in certain places without getting in trouble if certain criteria have been met.
Either way, where this is legal, it requires that the person be of the capacity to consent, which someone with dementia cannot do.