Not voting (in your election)

DNC members do not oppose “the left” as an electorate boogeyman. It opposes leftists when they act like an internal rival power center who come in shitting on Dems in office, demanding full control of the party, and acting like that should count as coalition building when it really hurts every vulnerable DNC member from the center leftward.

That does not mean the DNC is secretly closer to the GOP than to its own left flank; that claim is political anti-fanfic. Bernie lost, both times, because party insiders lined up against him and because he failed to win a broad enough coalition. I say this as someone who campaigned for him in 2016 and 2020 and have close friends who worked on his 2020 campaign in the PNW and then across the US.

And Mamdani, like AOC before him, undercut your case. They are evidence that the party establishment will adapt and move leftward, when you win inside Democratic primaries. It's called proving your electability. And as those progressive dems (and, hell, even democratic socialists) win races, build a durable voter base, and work with the party as a coalition, they'll find themselves with more internal influence.

“The DNC and GOP are basically the same" is defeatist slop. The Democratic coalition is a battlefield, and the left either gets strong enough to win on that field (either winning outright or through coalition building), or it resigns itself to irrelevance, because there is no serious alternative on the table.