There’s a real simple answer to that question: He or she won’t, because that’s impossible to do under current political conditions in the US.
(1) The massively disproportional political power of conservative old white people, because of The Wisdom of the Framers.
(2) The enormous status quo bias built into in the American system of governance, because of The Wisdom of the Framers.
(3) The absolute opposition of the nation’s economic, cultural, and media elites to any political changes that would cost them any money.
Joe Biden can’t do anything about this because his claims that Republicans will work with him are either delusional nonsense or strategic electoral pablum.
Bernie Sanders can’t do anything about this because his apparent belief that his election would also unleash some sort of left wing political tsunami is absurd on its face.
Elizabeth Warren doesn’t have a plan for this, because there’s no plan for this. Or rather the plan would have to be “have a better system and/or a better electorate,” which is a potentially good long-term plan but doesn’t work at all in the short term.
All this would be true even if Joe Manchin turns out to be the median vote in the Senate next year, which is an extremely optimistic scenario.
This election is incredibly important because it’s about stemming the tide of ethno-nationalism and fascism. Winning the presidency is absolutely critical to that in all sorts of ways (controlling the executive branch, foreign policy, judicial appointments etc.)
But it’s not about enacting anything even vaguely resembling a progressive political agenda, because in the short term that can’t happen. It’s about triage.
So ultimately I just can’t get too worked up about who the Democratic candidate for president is, because the ONLY thing that matters is defeating Trump, and winning enough down ballot races to stem the red tide. I have no idea who the best candidate is for those limited purposes and neither do you. So support the candidate you like best, but let’s keep our eye on the ball.