Information on campaign donations in federal elections.
FEC API is a free Government API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
Details from api.open.fec.gov.

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The FEC API is provided by OpenFEC API Documentation (api.open.fec.gov).
Yes. The FEC API is listed in the Government category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The FEC API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the FEC API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The FEC API is served over HTTPS, so it's safe to use in production.