(CNN) -- The National Federation of Independent Business is one of the most influential small-business advocacy groups in the country. They battle against government regulation, higher taxes and, perhaps most famously, Obamacare. And they do it all as the self-described "voice of small business."
But it turns out that the champions of Main Street America got more money last year from a group backed by billionaire industrialists Charles and David Koch than any other single source.
NFIB and its affiliated groups received $2.5 million from Freedom Partners Chamber of Commerce, a conservative advocacy group with deep ties to the Koch empire. Of the five men that sit on the group's board, four are current or former employees of Koch companies and one is a friend of Charles Koch's.
Freedom Partners gave the NFIB $1.5 million last year, the biggest single contribution the federation received, according to tax records. The Koch-backed group gave three other NFIB-affiliated group another $1 million, making Freedom Partners among the top two biggest contributors to those groups, records show.
The big-money donations are raising questions about whose agenda NFIB is serving, that of mom-and-pop businesses or the captains of big industry.
Lisa Gilbert of the government watchdog group Public Citizen said the NFIB is "taking the name of small business in vain."
"The idea that Koch brothers money in some way is going to help small businesses is laughable," Gilbert said. "What they're buying is the ability to help set the agenda."
And that agenda has included taking the lead on the lawsuit challenging Obamacare (NFIB v. Sebelius) that went to the U.S. Supreme Court. They also opposed increasing income tax rates on people making $250,000 or more, even though the vast majority of small businesses aren't affected by the rate hike.
NFIB spokeswoman Jean Card said the organization brings in the bulk of its revenue, about $86 million, from dues its 350,000 members pay. Members, she said, set the group's agenda.
Any suggestion to the contrary, is "simply not true."
"We do what our members tell us to do. We've been doing it since 1943," she said.
On taxes, the organization asked its members in 2010 if the federal government should extend all the Bush-era individual tax rates, 89% said yes. In November 2010 they asked members if Congress should repeal the Affordable Care Act and 93% said yes, Card said.
Freedom Partners advocates for lower taxes and supports NFIB's work on reducing government regulations, group spokesman James Davis said. Many of the group's more than 200 members started small businesses.
"Freedom Partners, many of the businesses that are represented -- and I can't prove this to you because I'm not going to give you a list of our businesses -- but they are small businesses," he said. "They aren't publicly traded companies."
Robert Tappan, a spokesman for Koch Industries, said the company's roots are as a small business.
"Koch supports small businesses all over the country through our relationships with our vendors and suppliers and we support wholeheartedly the fundamental mission of NFIB --principled entrepreneurship," he said.
This isn't the first time NFIB has taken money from big corporate interests. In 2011, NFIB took $850,000 from the nation's top insurance industry trade group as part of a campaign to repeal a key Obamacare tax. The back-channel transfer by America's Health Insurance Plans to the NFIB allowed insurers to fund a much more politically popular ally in the fight against a tax on health care premiums.