Political spending by labor unions exceeds their direct campaign contributions by a factor of four, according to the Wall Street Journal—vastly more than generally assumed:
Previous estimates have focused on labor unions' filings with federal election officials, which chronicle contributions made directly to federal candidates and union spending in support of candidates for Congress and the White House.
But unions spend far more money on a wider range of political activities, including supporting state and local candidates and deploying what has long been seen as the unions' most potent political weapon: persuading members to vote as unions want them to. …
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.