Who knows local Congressional districts the best?
The answer is pretty simple, but it's not what most people think. In order, here's who:
1. The voters in those districts. They're the ones who know what their districts want better than anyone.
2. Next in line are people who have already been elected to some political office in that district.
3. After that come the local party organization leadership folks.
Do you know who doesn't know jack-diddly about the local districts?
That would be the DNC and the DCCC. Typically, they know very little of value about those local districts.
What that means is that if you want to help Democrats win in any given district, donate directly to the Democratic candidate who wins the primary election. Your money will be better invested with them than with anyone else.
If you want to help the Democratic candidate in YOUR district, contact that candidate's campaign and ask how you can best help.