Democrats are out to make the Koch brothers the most infamous campaign donors in the 2014 election cycle.

But they are hardly the biggest spenders, nor is Americans for Prosperity, the 501(c)(4) they support, at the top of the list.

That distinction belongs to the nation's labor unions, whose ability to freely use worker dues to help favored candidates -- almost all of them Democrats -- puts them far above just about any other individual or group.

Labor union spending on both campaigns and lobbying can be difficult to track, however.

Unions are required to disclose donations to candidates and campaigns with the Federal Election Commission. According to the nonpartisan Center for Responsive Politics, that figure totaled $143 million during the 2011-2012 election cycle.

By comparison, Americans for Prosperity spent about $36 million on political activity during those two years, all of it spent against Democratic contenders, according to CRP.

But those numbers tell only part of the story.

A great deal of union spending on behalf of candidates includes not only direct donations but also spending on other activities that aid campaigns. Those efforts often include hiring workers to staff phone banks and staging get-out-the-vote efforts.

Those figures are reported to the Department of Labor. According to the National Institute for Labor Relations Research, a think tank critical of unions, the nation's largest labor groups reported to DOL that they spent a staggering $1.7 billion during the 2012 election cycle on electioneering and lobbying.

Stan Greer, a senior research associate with the think tank, said he believes the bulk of the money was spent on election work rather than lobbying.

“The Big Labor contribution is a large chunk of campaign spending,” Greer told the Washington Examiner. “And there is no one who is their equal in campaign politics.”

Not even the Kochs, who, according to CRP, rank 59th on its spending list, “Heavy Hitters: Top All-Time Donors, 1989-2014.”

The CRP list of top donors excludes 501(c)(4) groups such as Americans for Prosperity, which are not required to disclose their donors. But the list still highlights the gap between the Koch brothers' direct donations and those from labor groups.

The brothers’ corporation, Koch Industries, gave about $18 million, mostly to Republican candidates, during that time.

Six of the top 10 donors on the CRP list are labor unions, whose total contributions during the same time frame exceed $284 million.