Love ‘em or hate ‘em, tea party gatherings are the grassroots real deal. Organic, earnest groundswells of populist sentiment and full-throated political expression that have upset the political establishment left and right. Effective too, as they beat mainstream candidates from Delaware to Nevada to Alaska.

But just beneath the surface are professional fund-raisers, foundations, and political action committees – some of which have been around for years – pushing an agenda that neatly matches the conservative/libertarian aims of most tea partyers.

You can tell its effectiveness by the rhetoric of the opposition from across the political spectrum.

In Pictures: Tea Parties

Karl Rove (initially, at least) dissing tea party favorite Christine O’Donnell in her upset win in Delaware’s Republican senate primary. President Obama indirectly criticizing the insurgency’s well-funded lobbying in speeches to the Democratic faithful warning of the impact of the US Supreme Court’s Citizens United decision easing restrictions on how corporations may influence federal elections.

Changes in campaign finance law and the aims of the tea party movement converge in a mutually-supportive way.

Specifically, the tea party – particularly as it fights government spending and taxation under Obama and the Democratically-controlled Congress – is the sometimes-raucous face of traditional limited government lobbying. In return, the movement’s efforts get significant financial backing from those lobbying organizations, which in turn are supported by conservative foundations and business interests.

The Koch brothers

For example, in a recent investigative report in the New Yorker magazine, Jane Mayer details the links between billionaire industrialists David and Charles Koch and the tea party movement. She writes:

"By giving money to 'educate,' fund, and organize Tea Party protesters, they have helped turn their private agenda into a mass movement. Bruce Bartlett, a conservative economist and a historian, who once worked at the National Center for Policy Analysis, a Dallas-based think tank that the Kochs fund, said, 'The problem with the whole libertarian movement is that it’s been all chiefs and no Indians. There haven’t been any actual people, like voters, who [care] about it. So the problem for the Kochs has been trying to create a movement.' With the emergence of the Tea Party, he said, 'everyone suddenly sees that for the first time there are Indians out there – people who can provide real ideological power.' The Kochs, he said, are 'trying to shape and control and channel the populist uprising into their own policies.'"

The money, Mayer and others report, comes through such conservative organizations as Americans for Prosperity and FreedomWorks.

Another major source of tea party funding is the Tea Party Express, which poured hundreds of thousands of dollars into the successful GOP primary senate campaigns of Christine O’Donnell in Delaware and Joe Miller in Alaska.

Based in Sacramento, Calif., the Tea Party Express is run by Sal Russo, a Republican fund raiser and public relations guru who began his career working for Ronald Reagan. Russo is also the chief strategist for “Our Country Deserves Better,” a PAC formed to defeat Obama in the 2008 presidential election.

“As a pivotal player in the ‘tea party’ movement, Russo has helped drive its cause by raising millions of dollars and crafting caustic ads about its opponents,” reports the Los Angeles Times. “There's no question that Tea Party Express, the political action committee Russo runs out of his Sacramento-based firm, is the advertising muscle behind the tea party insurgency.... As the only tea party group making significant advertising buys, Tea Party Express has become one of the most potent forces in the protest movement.”

Millions from the Tea Party Express

Our Country Deserves Better, which launched the first Tea Party Express bus tour last year, raised and spent just over $1 million in the 2008 campaign year. So far in 2010, it’s raised and spent more than $5 million. Large chunks of that went to the GOP primary campaigns of tea party favorites Christine O’Donnell ($237,000) and Joe Miller (nearly $600,000).

The problem for those trying to ferret out where the money comes from – and for Obama and Democrats as they seek to toughen campaign finance reporting in the wake of the Citizens United court decision – is that it’s getting harder to do so.

“Federal campaign spending by groups other than candidates and parties this election cycle has far outpaced similar spending from the last midterm election and could rival the 2008 presidential campaign,” the New York Times reports. “But with recent decisions by the Supreme Court and the Federal Elections Commission, it has become harder to know whose dollars they are.”

This chart shows that the percentage of such groups involved in “electioneering communications” and reporting the names of their donors has dropped from 98 percent in 2004 to 32 percent this year.

The tea party may have sturdy, spreading grassroots. But there’s plenty of special-interest money behind it as well.

In Pictures: Tea Parties