Organized labor spends about four times as much on politics and lobbying as generally thought, according to a Wall Street Journal analysis, a finding that shines a light on an aspect of labor's political activity that has often been overlooked.

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 new figures come from a little-known set of annual reports to the Labor Department in which local unions, their national parents and labor federations have been required to detail their spending on politics and lobbying since 2005.

This kind of spending, which is on the rise, has enabled the largest unions to maintain and in some cases increase their clout in Washington and state capitals, even though unionized workers make up a declining share of the workforce. The result is that labor could be a stronger counterweight than commonly realized to "super PACs" that today raise millions from wealthy donors, in many cases to support Republican candidates and causes.