The health care industry contributes substantially to both parties, even though the Democratic party supports single-payer. They contributed more to the GOP in the 2000s, but the GOP controlled the government for most of that time and held the presidency. Starting in 2008, health care resumed contributing more to the Democrats, as they did at the start of the Clinton administration.
The idea that politicians assume public policy positions because they're captured by donors is sometimes true, but it's incorrect as often as not.
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.
If American politicians sported their sponsors logos like nascar drivers do, things would become much clearer.