Are you using Twitch pubsub to deliver data from your backend to your frontend? If not, you should consider doing so.
Depending on how many messages you expect your frontend to send to your backend, I am not convinced that websockets are the correct technology. If you expect to have your frontend send multiple messages without requiring setup/teardown of the socket, this could be worthwhile.
Be careful of scaling a node backend. Ensure that your requests are just persisting data, and returning, with another consumer handling aggregation.
Ask yourself questions like: will my backend support 500k viewers voting at once? How about 1million?