Political campaigns have figured out what direct marketers have long known: There’s good money in your email address and contact information.

During the past four years, more than 90 campaigns, parties and political action committees have sold their donors’ personal contact information to outside groups as a way to raise millions of dollars, according to a Roll Call study.

Though privacy advocates criticize the practice, campaign records show that campaigns have paid about $50 million since 2007 to rent donor lists, though this includes purchases from some private firms as well as campaigns.

Some of the campaigns that have charged the most money for access to their donors’ information are the biggest names from the 2008 presidential election, including Secretary of State Hillary Rodham Clinton, Sen. John McCain (R-Ariz.) and President Barack Obama.

“These lists are the biggest assets of any campaign,” said Caleb Burns, a partner at Wiley Rein in the firm’s election law and government ethics section. “If you think about campaigns, they are largely ephemeral.”

Clinton’s campaign disclosed receipts topping $4 million from the rental of her list to more than 60 campaigns, organizations and list brokers since 2008. Some of the campaigns paying the most for this contact information included $66,000 from Sen. Barbara Boxer (D-Calif.), $47,000 from Sen. Kirsten Gillibrand (D-N.Y.) and almost $24,000 from Sen. Charles Schumer (D-N.Y.).

Her campaign also rented this information to causes connected to the former Senator, including the William J. Clinton Foundation with almost $350,000 and her former leadership fund, HillPAC, which has paid $822,000 for the list throughout the years. Under campaign finance rules, the campaign’s donor list is an asset that she cannot simply give away to groups or campaigns — they have to pay to rent it.

The infusion of funds from rental fees was badly needed by Clinton’s presidential campaign because it had $5.9 million in debt at the end of the 2008 election cycle. The campaign has since lowered this debt to $274,000 through the end of September.

McCain’s presidential campaign also raised significant funds by renting its list to campaigns for lawmakers such as Rep. Michele Bachmann (R-Minn.) and GOP Senate candidates Carly Fiorina in California and Joseph DioGuardi in New York.

Technically, the former Republican presidential nominee leads all campaigns with more than $4.6 million in rental fees listed in FEC reports since 2008. But the lion’s share of this money — about $4.5 million — came from his various campaigns renting lists to McCain’s affiliated organizations, including his political compliance fund, his joint fundraising committee and his leadership PAC. Similar transactions show Obama’s presidential campaign received more than $1.3 million from the Democratic National Committee in April to rent contact information.

Several Republican candidates rented lists from Mitt Romney’s presidential campaign since 2008, totaling more than $150,000, according to Federal Election Commission records. Some of these Republican campaigns that reported renting Romney’s list include former Rep. Tom Feeney (Fla.), Rep. Mary Bono Mack (Calif.) and her husband, Rep. Connie Mack IV (R-Fla.), as well as Romney’s leadership PAC, Free and Strong America PAC.

Some of the Congressional campaigns that are receiving the most money from selling their lists include those of Sen. John Kerry (D–Mass.), with more than $273,000, and Sen. Al Franken (D–Minn.), with $91,000.

Kerry’s staff clarified that the campaign limits the rental of its donor list “to like-minded organizations such as the DNC and DSCC” and points out that it provided only postal addresses, not emails.

“This is the best way of helping progressive organizations, and Democratic candidates — we rent the lists as opposed to giving the information because under the law, it would be considered an in-kind contribution and thus would be capped,” Jodi Seth, Kerry’s communications director, wrote in an email. “It also helps cover the overhead of maintaining our lists.”

Kerry and Franken used an outside vendor — called a list broker — as a middleman to rent the contact information to other campaigns. The sale and management of these lists has brought in millions of dollars for organizations such as Pinnacle List Co., Preferred Communications and Legacy Lists Marketing.

These companies advertise on their websites available databases of donor information from a wide range of campaigns, including 7,300 contributors to Rep. Joe Wilson (R-S.C.), 31,800 “active donors” to Sen. Bernie Sanders (I-Vt.) and 307,700 contacts from the Democratic Congressional Campaign Committee.

A popular list broker for Republicans is Capitol Hill Lists in Georgia, which has paid $111,000 to Sen. David Vitter (La.), $83,000 to Sen. Marco Rubio (Fla.) and $45,000 to Sen. James Risch (Idaho) since 2009 for list information.

While renting these lists might be lucrative, privacy advocates say campaigns should inform potential contributors that they may sell their contact information to others.

“This wholesale trade in donor information violates people’s privacy,” said Lillie Coney, associate director of the Electronic Privacy Information Center. Coney said the public should be aware that list brokers are selling contact information to sources that range from political campaigns to magazines.

“We shouldn’t have to give up our privacy just to exercise our right to political expression,” said J. Bradley Jansen, director of the Center for Financial Privacy and Human Rights.

Jansen is against the distribution and rental of donor contact information. But instead of laying blame with the candidates, he said the problem can be traced to campaign finance disclosure regulations and court decisions ruling that Americans do not have an expectation of privacy from third parties.

Jansen’s contact information was likely sold recently after making $2,100 in donations to Republican presidential candidate Rep. Ron Paul (Texas) in 2007. FEC records show Paul later rented his donor list to his son, Sen. Rand Paul (R), for $4,600 during his successful 2010 race in Kentucky.

But Jansen did not object to his name and others being rented by Paul’s son, saying, “They are in the same family by blood and ideology.”

Rep. Paul’s staff explained that he has a policy of not renting his donor list but made an exception in this case with his son.

“Dr. Paul is deeply committed to protecting the privacy of his donors, and has a strict policy against renting his lists,” spokesman Jesse Benton said in an email. “Dr. Paul has some of the largest and most valuable lists in the political universe, and receives regular offers to sell or rent them for hundreds of thousands of dollars. He has only rented his list to his son, Senator Paul.”

While purchasing lists of campaign donors is common, some lawmakers are choosing to stay out of the fray of this lucrative process.

“I know a fair number of candidates who don’t rent out their lists,” Burns said, “because they don’t want their donors tapped into and otherwise cannibalized by other political campaigns.”