tl;dr Think of this as a mutual fund investing in electing progressives up and down the ticket.
Phillosophy. I don't know which organizations will have the biggest impact, and neither does anybody else. So for each priority, I'm supporting several organizations that are taking different approaches. Where possible, I focus on data-driven groups.
Voter turnout: Funds will go to Vote Forward, People's Action, and Indivisible. Vote Forward turns out infrequent (Democratic) voters. People's Action uses "deep canvassing" to try to win hearts and minds. Both organizations run randomized experiments to hone their methods and publish results. Indivisible is, of course, probably the biggest, most active progressive organization around.
Protect the Vote: We can't turn out our voters if they aren't allowed to vote. Funds will support Common Cause and Fair Fight. Common Cause has been behind some of the most important voting rights lawsuits in recent years and supports democratic reforms more generally. Fair Fight was launched by Stacey Abrams after GA suppressed enough Democratic ballots to prevent her election.
Flipping State Houses: Because of redistricting, state elections this year will determine state and federal elections for the next decade. Funds will go to the state election funds for the MI, PA, and GA Democratic Parties, as well as the non-federal Flippable State Fund (non-federal). MI and PA are just a couple seats away from flipping Blue. GA is a longer (but achievable) shot, and mobilizing voters will help impact the two open senate races. Flippable distributes funds to state-level candidates based on an algorithm predicting where each dollar will have the most impact on flipping a state house.
Flipping the Senate & Presidency: Funds will go to the federal Flippable States Fund and to Joe Biden. Flippable redistributes to the Congressional candidates, optimizing how likely each dollar is to flip a seat. Joe Biden ... I'm not a fan, but none of the rest of this matters if Trump stays in office.