Locate the Senator or Assembly member responsible for authoring the proposed legislation. Information of note: party affiliation (Republican, Democrat, etc.), voting record, campaign donations, and contact information.
Most bills introduced in Congress will have supporters in the form of corporations, financial institutions, labor unions, professional associations, educational groups, medical interests, and various public interest and social issue groups. These supporters hire lobbyists to influence the drafting and passing of legislation on behalf of their clients.
When you're looking for what companies, organizations, or groups support a bill, it can be frustrating combing through the thousands of search results and documents related to the legislation you're researching. However, organizations like Propublica and The Center for Responsive Politics (CRP) have created tools that serve as helpful starting points for researching who is lobbying for what.
The client is the organization, company, or person that hired lobbyists to advocate for their interests. Your research can take form in a simple Google search, but you will likely need to do more digging, using information you discover along the way, to search in individual websites and databases. Take the following example as one way to begin.
Say you're researching the bill, H.R. 1296, the Assault Weapons Ban of 2019.
Head to ProPublica's lobbying website and type in "Assault Weapons Ban of 2019." See the image below.
You'll get a list of potential lobbyists and their clients. You'll have to go in to view the Details of the issues to confirm that the bill was one of those targeted by the lobbyists/clients (see the image below). Some of the results aren't related to the specific bill H.R. 1296, but to the issue the bill is focused on. These results may still be helpful, though, as you can gather a broad idea of the kinds of groups who have supported or opposed similar legislation in the past.
From there, you can continue to research information about the client(s). For example, looking at the image above, the client, The March for Our Lives Action Fund, is a prominent organization, and it will be fairly easy to find information about it and its advocacy online. However, there are other useful and powerful search tools you can use to find credible information. Start at the Arguing For or Against a Bill section to begin exploring some of these.