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If your email inbox starts overflowing with messages from political campaigns this election season, it could be because your state sold you out.

A Fox News study has found 19 states plus the District of Columbia, now ask for an email address on voter registration cards. In nine of those states, email addresses from the cards are then sold to political parties, organizing groups, lawmakers and campaigns who can use them to send unsolicited emails.

If it were a Viagra ad, it be considered a crime in some states. But a political message, that's all perfectly legal.

The CAN-SPAM (Controlling the Assault of Non-Solicited Pornography and Marketing) law enacted in 2003 puts restrictions on commercial mass emailing, but not on political mass emailing. Politicians can "spam" and do. Political messages of any kind, including electronic, are protected free speech under the First Amendment.

"Political communications are not spam. Political communications are a demonstration of free speech in America," said Stuart Shapiro, president of iConstituent, a Washington, D.C.-based firm which uses state-generated email lists to send messages on behalf of clients on all sides of the political spectrum.

"There is a tenet in government that is based on communicating with our constituents, and email is one of the most effective ways to do it," Shapiro said. "People look forward to it and want it."

"Politicians love the fact that their perceived freedom of speech is more important than voters' privacy," said Shaun Dakin, president and CEO of The National Political Do Not Contact Registry, a non-profit voters' rights advocacy group based in Falls Church, Va.

Dakin said his organization has received hundreds of thousands of complaints from voters who are fed up with political groups and candidates invading their privacy via phone calls and emails. He said he expects complaints to climb as more states start asking for email addresses.

"With the advent of the Internet and smartphones, iPhones and iPads, everybody is using electronic communications. States are looking to get those email addresses and not being explicit about how those email addresses are being used," he said.

"It just seems to be duplicitous. When people sign up for voter registration, they're just signing up because they want to vote."

Richard Michael, an Internet campaign advisor at GrassrootsPhonebank.com in Los Angeles, said email addresses "are fair game, no matter how you get them."

"If someone puts their email on a voter registration, it's reasonable to assume they are amenable to receiving political messages," Michael said.

Like phone numbers, email addresses are not required to register to vote anywhere in the United States. Giving the information is optional, but that may not be clear to the average voter.

"I think this is really one of those untold stories. It's all going on behind the scenes," said Kim Alexander, president of The California Voter Foundation, a nonprofit organization which produced the study "Voter Privacy in the Digital Age."

"People who are in the election business, people who are administrators, people who are in campaigns, they all know what's going on. The voters are in the dark, and that has got to change. It's disrespectful and it's deceitful," said Alexander.

In 2008, Alexander helped redesign California's voter registration card to clearly mark the word "optional" in the form's email field.

Eight of the states that collect emails fail to clearly mark the information as "optional" on their registration forms. Not one of the states that sells email addresses clearly explains on the voter form that emails could or would be sold.

"States are now getting into, essentially, the data brokering business so they understand that the more points of contact that they have for a voter, they can make more money," Dakin said.

But Shapiro said that's an inaccurate depiction of the states' role.

"I don't think it is fair to portray that states are selling voter files or selling voter email addresses," Shapiro said. "What they're doing is they're collecting voter information, collecting email information. They're providing that information in a wholesome and legal way to those parties that have the legal right to use them. "

In fact, in 47 of the 50 states, voter registration data is sold at either the state or county level -- and it's nothing new. Mining registration rolls for voter information is part of the election process.

"Long before the Internet came along, long before identity theft became an issue, campaigns have been collecting data on voters and creating voter profiles," Alexander said. "Campaigns use this so they can target their messages more effectively to voters -- to say these people are environmentalists, these people support gun rights. They will often merge voter data with other data to create profiles to help them more precisely target their message to particular voters."

Personal information about registered voters can be invaluable information to campaigns, but selling it is not a big moneymaker for states.

A few states make it available for free, considering it a matter of public record. Some sell a statewide list of every voters' name, address and date of birth for as little as $25. Others offer regularly updated subscriptions for $5,000 per year. The most expensive statewide list Fox News found is in Wisconsin, at $12,500, which includes emails.

Except for Iowa and Oregon, which charge a nominal extra fee, most states that sell email addresses include them in the price of a basic voter roll.

Right now, the number of email addresses states have collected from voter registration forms is not extraordinarily high. For example, of Oregon's 2.5 million voters, only 20,000 emails are in the state's database.

But official state email collections are expected to grow for a number of reasons. Shapiro says email addresses are becoming more stable than phone numbers. As people move and re-register, they are ditching their landlines but keeping their emails.

Then there are 18-year-old new voters who may be more apt to share their email addresses because they don't remember life without one. Finally, there is online voter registration. Eight states currently allow it, and providing an email is part of the process. Emails are also often used to access absentee ballots for overseas and military voting.

Even though political email lists are growing, Dakin says the messages themselves should not be restricted by law the way commercial emails are. He says a system needs to be in place to balance freedom of speech and the right to privacy.

"I think it would be best for the (Republican National Committee) and (Democratic National Committee) to manage a Democratic Do-Not-Call, Do-Not-Email, Do-Not-Spam list and a Republican Do-Not-Call, Do-Not-Spam, Do-Not-Email list. If they could do that, and coordinate with the variety of political action committees and unions and 527s, that would be very helpful. But right now, there is no one central database for doing any of this stuff," Dakin said.

Receivers of political emails do have the right to opt-out from lists -- they just can't do it in one click. Instead, they must do it with every email they receive, clicking on an "unsubscribe" link, if the email has one or by replying to the sender with "Remove Me" request.

Shapiro says few people actually do this.

"iConstituent, last year, probably mailed more than a billion email records out throughout all of America for Congress for various other legislators and we have a very, very low unsubscribe rate. It is well under one-tenth of 1 percent."

Shapiro says this proves people treat political emails differently from commercial spam.

Other political marketers say that may not be the case -- instead the email is being marked as spam. Email users who don't want to receive political emails can use the mark-as-spam button to report bulk senders to their Internet service provider.

That can hurt both the client and the business. That's why Manuel Romero, president of the marketing company SimpleSend, does not allow his political clients to send email messages to voter registration lists anymore.

"We got burned," he said. Romero's company was reported by people whose email addresses were purchased from California counties and legitimately used.

"It's legal," Romero said, "But in the real world, few people know that."

States that ask for email addresses on voter registration forms:

Arizona, Arkansas, California, Colorado, District of Columbia, Delaware, Indiana, Iowa, Maryland, Minnesota, Montana, Nebraska, New Jersey, Oregon, Rhode Island, Tennessee, Virginia, Washington, Wisconsin and Wyoming

States that sell email addresses listed on voter registration forms:

Arkansas, California, Indiana, Iowa, Montana, Oregon, New Jersey, Rhode Island and Wisconsin