If you're an American voter and have provided personal information to a company, chances are data groups have shared it with political parties to help them target potential supporters.

One of the main players is NGP VAN, which manages the Democratic National Committee's database.

Its name recently surfaced in connection with a data breach blamed on a technical glitch that enabled the campaign of presidential hopeful Bernie Sanders to access voter data belonging to rival Hillary Clinton.

The incident raises questions about the reach of the database.

"Everyone" is in there, Kevin Thurman, who served as Clinton's deputy Internet director during her last campaign in 2008, said with a laugh.

"Every voter in America, since 2004, dead or alive."

NGP VAN estimates the number at about 195 million people, far more than the 146 million currently registered as voters. Voter registration is not mandatory in the United States.

The database even includes adults who don't vote but who the campaigns hope to convince to turn out ahead of the November presidential election.

All Democratic candidates -- be they involved in local elections or a major presidential campaign -- use NGP VAN's data.

The group helps candidates better target potential voters by avoiding those who will probably never cast a ballot for them, thereby reducing campaign expenses.

Other companies provide similar services, including for Republicans.

"If you're Trump, you're not going to lose time targeting the Clinton list," explained Brandi Travis, spokeswoman for NGP VAN competitor Aristotle.

- Companies sell data -

The Democratic Party has been collecting information on American voters since 2004.

The data -- which includes a registered voter's age, address and history of election participation -- is based on publicly available information.

Through brokers, the party then buys information that private companies own on their clients. CVS pharmacies, for example, sell this information.

The National Rifle Association, America's leading gun lobby, also shares data on its members.

"They buy information from corporate databases... or newspaper subscriptions," Thurman explained.

Clients may not necessarily be aware of this practice, which is legal.

"They want to know what type of product you bought," said Sasha Issenberg, a journalist who has published a book, "The Victory Lab," on the issue.

These groups also sell their clients' email address.

With NGP VAN's information, the Democratic Party can also learn about someone's educational history and, often, his or her ethnic background.

Then comes the analysis.

A 30-year-old woman living in a major city and subscribed to a literary magazine, for example, has more chances of voting for Clinton than a 60-year-old man living in a southern rural area and subscribed to a magazine for weapons enthusiasts.

When they go knocking on doors, volunteers can then further refine the data by asking whether the targeted voter cares more, say, about climate change or ending unemployment.

Among NGP VAN's dozens of competitors, some, like Aristotle, build their own databases.

NationBuilder told AFP that French political parties have sought its services ahead of next year's presidential election in France.

The Republicans have a similar database.

- 'I agree' -

Data collectors stress that all information is obtained legally, even if Americans are often unaware of the extent of parties' knowledge of details of their personal lives.

"When you click the 'I agree' button online, you don't read the text, but you accept that your information can be sold," said Joe Curran, founder of Filpac, which works with the Republicans.

"It's public, but people just don't know it."

Thurman agreed -- people simply don't realize what they've signed up for when they register on a website.

"There is no way our information can compete with the information the private companies have on Americans," Thurman added.

"Compared to them, we are weak."