What holds back the Democratic Party, really?

Here's a hint - it isn't progressive policies -

Clinton Democrats or followers of the Third Way still have the power, and that is part of what is holding back an agenda for working people. They stand in the way of a movement that believes the country should move away from destructive corporate politics, and until their obstruction is overcome, progress on economic, environmental, racial, and social justice will be exceptionally difficult to achieve.

https://shadowproof.com/2018/07/25/clinton-democrats-embrace-losing-strategy-combat-sanders-style-socialism-midterms/