First, in a two party system you have a broad spectrum of people in each party. The dems range from left to center, the republicans from right to far right.
Which is to say, it's impossible to get absolute agreements in either of these parties(Hell, you see that playing out on the Republican side right this minute, crippling their ability to get their big ticket legislation through). Precisely because of this you're not gonna see an absolute accord on issues, though they'll at least agree on the problems and that something needs to be done about them. That being said. if we're talking leadership, Pelosi has supported single payer repeatedly over her career, Bill Clinton wanted universal healthcare, Obama ran on having a public option, Harry Reid saw Obamacare as the first step toward Single Payer, which he supported.
Then you have the dems in california passing single payer...
So... Tell me how this makes the party against a public option?
Sourcing:
https://www.forbes.com/sites/theapo...ep-toward-a-single-payer-system/#d464f063af9d