Parties don't matter, people do. They only need to contribute to a few key people to influence lawmaking. A significant number of politicians (of both parties

Parties don't matter, people do. They only need to contribute to a few key people to influence lawmaking. A significant number of politicians (of both parties) will oppose single-payer on principle. A significant number will oppose it based on what they think their electorate wants. Some will vote for single-payer on principle. And some are on the fence or just don't care. Lobbyists are paid big money to ferret out these few and influence them.