This plain-looking chart tells much about the complex state of campaign finance laws in federal elections.

Put together by Wesleyan Media Project and the Center for Responsive Politics, the chart shows how much is not known about political ad spending in today's campaign finance landscape. It details the television ad spending by 12 prominent political groups, including President Obama's Super PAC Priorities USA Action, and Karl Rove's two-headed conservative group, Crossroads GPS and American Crossroads.

By this sample, more than two-thirds of all political ad spending goes unreported.

That's because there are many types of campaign ads, which may seem the same, but are treated much differently under campaign finance law. One type, called issue ads, doesn't explicitly tell viewers to vote for or against a candidate – but almost always does so implicitly.

An issue ad might connect a candidate to an unpopular policy or criticize a candidate's views on a particular subject, but as long it does not ask the viewer to vote accordingly, it remains "issue advocacy". As such, it is only reported as election spending when run within 30 days of a primary or 60 days of a general election.

At all other times, the ads are not reported to the Federal Election Commission. The only way to know exactly how much and how often a group ran issue ads is to track down the relevant paperwork at the local TV stations where the ads ran. In this chart, Wesleyan Media Project estimated the costs of ads based on when and where the ads were run.

Below are two examples of issue ads which, if broadcasted before September 7, will not be reported to the FEC.

From Priorities USA Action, Obama's Super PAC:

From American Crossroads, the Karl Rove-linked conservative Super PAC: