The Koch brothers poured $301 million into politically-active groups in 2012, according to a report from the Center for Responsive Politics.

And of the $170 million in political spending reported by those groups to the Federal Election Commission, CRP estimates about $86 million came from Koch-linked organizations.

That $86 million totals more than a quarter of all "dark money" spending, from groups that do not have to directly disclose their donors, in 2012.

And the Kochs' electoral reach was wide in 2012. The second-most politically active nonprofit, in terms of dollars spent, was Americans for Prosperity, the Kochs' primary nonprofit.

That group received more than a third of its contributions from three other Koch-linked dark money groups: The Center to Protect Patient Rights, Freedom Partners, and TC4 Trust.

The report outlines the myriad ways in which contributions are funneled through various groups down to local-level political action committees that engage in elections, ballot initiatives and other political activities.

The practice is not unique to Republican groups or to the Koch brothers. But the rise of dark money groups since the Citizens United Supreme Court decision has inspired greater scrutiny over the role of money in politics, and prompted scattered efforts to reform the system.

Most recently, the IRS and the Treasury Department issued proposed regulations to curtail the political activity that 501(c)(4) groups can engage in and still retain their nonprofit status.