Seconding Center for American Progress. There are also a ton of issue specific organizations and professional societies that are non partisan, but focus on improving policies that tend to be left leaning. For example Planned Parenthood is technically a nonpartisan nonprofit, but many other health or environment related non-profits have a policy team that tries to advance their cause they best they know how. Being nonpartisan means they aren’t limited to speaking with one side or the other and can keep a drum beat going on their issues regardless of the political winds. These orgs could use your donations now more than ever.