I'm happy to specify completely, actually, I just figured a general question would lead to answers that are more useful to the community.

In my case, I'm helping to set up an organization to divert money away from major party U.S. campaign funds and to efficient charities. The idea is that if I donate $100 to the Democratic Party, and you donate $200 to the Republican party (or to their nominees for President, say), the net marginal effect on the election is very similar to if you'd donated $100 and I've donated nothing; $100 from each of us is being canceled out. So we're going to make a site where people can donate to either of two opposing causes, we'll hold it in escrow for a little, and then at a preset time the money that would be canceling out goes to a GiveWell charity instead. So if we get $5000 in donations for the Democrats and $2000 for Republicans, the Democrats get $3000 and the neutral charity gets $4000. From an individual donor's point of view, each dollar you donate will either become a dollar for your side, or take away a dollar from the opposing side.

This obviously steps into a lot of election law, so that's probably the expertise I'll be looking for. We also need to figure out what type of organization(s) we need to be: it seems ideal to incorporate as a 501c(3) just so that people can make tax-deductible donations to us (whether donations made through us that end up going to charity can be tax-deductible is another issue). I think the spirit of the regulations should permit that, but I am not a lawyer and I've heard conflicting opinions on whether the letter of the law does.

And those issues aside, I feel like there could be more legal gotchas that I'm not anticipating to do with Handling Other People's Money.