I have seen some messages lately from my district representative that are essentially talking points of a political lobbying group. I would like to find out if and how much money he has received from this lobbying group.
Where can I find a reliable resource that shows all donations to a politician?
If you looking in the USA, the source of data would ultimately come from legally required campaign finance and financial disclosures filed with the FEC (Federal Elections Commission).
FEC: Campaign Finance Disclosure Search
However, things get murkier with assorted PACS, "dark money" and third party "independent" expenditures.
One particular group that tries to consolidate that information and give a more complete picture is the Center for Responsive Politics, a non-partisan group dedicated to a more transparent process. They not only have good resources regarding how everything works, but they do data consolidation and analysis that you can sort and customize in a number of different ways (what industries contributed most heavily, what percentage of donations fell into particular donation size tiers, etc).
PoloHoleSet's answer covers the federal level. Below that, individual states usually have their own organizations for tracking this data and making it available. They will often track this kind of data for publicly elected officials at the state level and below, such as counties and cities.
An example is the Washington State Public Disclosure Commission.
You'll have to search for your own state's equivalent organization.
