I think the best thing you could do with that amount of money would be to start a community organizing non-profit. Things like registering voters, tenant advocacy, helping people gain access to benefits, etc. etc. Basically, just organizing and connecting people to the system.

If you’re restricting things to just electoral politics, though, I’d say focus on close elections at the state level. A half billion dollars isn’t much in the context of a modern presidential election but it’s an absurd amount for state legislature elections. You’re probably not going to turn Mississippi blue or Massachusetts red (or whatever your goal is) but control of a swingy state like Virginia usually comes down to a handful of close elections. You could probably make a meaningful difference in 20 states just with get out the vote funding.