The LM-2 is a financial disclosure form many unions are required to file with the federal government annually. The form provides information about union revenue, membership numbers, political spending, reimbursements, the compensation of union leaders, grants, spending on representation and more. The information is much more detailed than what you would get from federal survey data or non-profit 990 filings. For that reason, the form can be used to track general trends, union membership numbers, or provide oversight of union spending.
The tutorial video and document on this page provide a brief overview explaining what LM-2s are, how to interpret them, and how they