The scale of Freedom Partners’ fund-raising is striking: It raised $256 million between November 2011 and last year’s election, according to the returns, details of which were reported on Thursday by Politico. That rivals or exceeds the annual budgets of the largest advocacy groups in the nation, like the National Rifle Association and the U.S. Chamber of Commerce.

But the returns also reflect a significant shift in the tax strategies the Koch operation deploys to avoid challenge from the Internal Revenue Service, which limits how much nonprofit groups can spend to aid or defeat candidates.

Other donor clearinghouses, along with nearly all of the political groups they support, register with the I.R.S. as “social welfare” groups under Section 501(c)4 of the tax code. That has let such groups spend money on elections while keeping their donors secret — drawing increasing regulatory and legislative scrutiny from critics who assert that some of the groups are violating campaign laws.

But Freedom Partners established itself in November 2011 as a 501(c)6 “business league,” typically a trade association of corporations, like the Chamber of Commerce, organized to promote a common business interest. Instead of donors, it has more than 200 “members,” each making a minimum $100,000 contribution, which Freedom Partners classifies as member dues. The approach gives it many of the same advantages social welfare groups have, with one significant addition: Some contributions to the group may be tax deductible as business expenses.