The usual measure of unions' clout encompasses chiefly what they spend supporting federal candidates through their political-action committees, which are funded with voluntary contributions, and lobbying Washington, which is a cost borne by the unions' own coffers.
These kinds of spending, which unions report to the Federal Election Commission and to Congress, totaled $1.1 billion from 2005 through 2011, according to the nonpartisan Center for Responsive Politics.
The unions' reports to the Labor Department capture an additional $3.3 billion that unions spent over the same period on political activity.