As the 2010 midterm elections approach, the Center for Public Integrity sought to determine who bankrolls Washington’s most powerful lawmakers and why — not just recently, but over the entire course of their federal careers. We looked hard at the big money behind the four top congressional leaders: Speaker of the House Nancy Pelosi, House Minority Leader John Boehner, Senate Majority Leader Harry Reid, and Senate Minority Leader Mitch McConnell. Through analysis of three decades worth of CQ MoneyLine records on contributions to all of their campaign committees and leadership PACs, we calculated the ten top PAC donors and five top individual contributors to each of the leadership quartet. Where companies and associations had merged, we combined all PACs affiliated with parts of the current entities. We combed through lobbying disclosure forms, press releases, news stories, and voting records to see whether the legislators vigorously backed the agendas of their top patrons. The answer: a definite yes.

Beyond that, some common themes emerged.

The two Democratic leaders leaned heavily on labor unions and trial lawyers. Eight of Congresswoman Pelosi’s top ten PAC supporters and five of Senator Reid’s represent those interests. Only one labor union made Congressman Boehner’s top ten list and none were on Senator McConnell’s ledger. By contrast, the Republican leaders’ most generous backers represented corporations and trade associations: nine of Boehner’s top ten and all ten top McConnell PAC donors represented business interests. Just five such groups made Reid’s list and two graced Pelosi’s.

The single top career backer of Boehner, McConnell, and Reid, however, was the same: telecom giant AT&T. This included contributions from companies like BellSouth, Cingular, and SBC that are now part of the AT&T family. Between just these three politicians, AT&T’s PACs contributed more than $525,000 in campaign cash. This bipartisan investment has paid dividends over the years as the trio has backed the firm’s priorities on telecommunications legislation and played key roles in granting the company immunity for its participation in the George W. Bush administration’s warrantless wiretapping program.

The financial services industry has also been a bipartisan player of significant heft: banking behemoth JPMorgan Chase’s affiliated PACs appeared on the top ten lists of three of the four leaders, rival Bank of America appears on two, and the industry’s American Bankers Association also appears on two of the four lists. All four of the leaders backed the Emergency Economic Stabilization Act of 2008, the controversial government bailout of the financial sector.

Republicans also received a great deal of money from the tobacco industry. Altria placed in the top ten for both Boehner (at least $181,959) and McConnell ($153,500 or more); Reynolds American also appeared on McConnell’s list (at least $101,000). Altria also appeared in the top-ten list for Reid, even though he has denounced cigarettes as “poisonous products.” The company’s PACs gave the Nevada Democrat at least $93,900. Only Pelosi escaped the tobacco money addiction, listing just a single $1,000 contribution from Altria’s Philip Morris PAC, way back in 1987.

The top individual donors tended to be much more varied. Many were longtime personal friends of the elected officials. Several were wealthy business executives and lobbyists whose interests were clearly furthered by the leaders’ actions. But those we spoke to all had one thing in common: each assured us that their contributions were not meant to gain access or influence.

Meredith McGehee, policy director at the Campaign Legal Center, told the Center for Public Integrity that “on some level the correlation between the interests of donors and the actions of Members of Congress is as old as the Republic. The explanation of that correlation has its roots in two camps: first, donors prefer to give to politicians they agree with. Second, politicians like to please their donors to ensure they keep their coffers open for future needs.” She notes broad disagreement between campaign finance reform supporters and opponents as to where the lines cross, but called the status quo “a pay-to-play system” in which elected officials are often “the last to fully comprehend their own motivations and to know when these two motivations dovetail.”