Make it so the Democratic Party is full of progressives, socialists, social democrats, whatever your favorite brand of left-wing politics is.
The problem is that they'll, again, fight you every step of the way. An ideological takeover will have to happen over their dead bodies, and meanwhile they'll keep demanding concessions so they don't expel you from the party and disallow you from running for primaries. The crux of the issue with the ideological takeover route is that this contradiction will lose you legitimacy in the eyes of your supporters as you're forced into compromise after compromise in order not to alienate the neoliberals, and they'll give you fucking nothing in return.
Ross Perot couldn't pull it off despite spending about 80% what the major parties spent and capturing nearly 19% of votes in a vastly more friendly media environment.
Uh... obviously you can't win an election with only 19% of the vote?