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Caging is a pejorative term used to describe a certain type of voter suppression. The name derives from a benign practice in direct mail fundraising where organizations that send out fundraising solicitations enter into a contract with an outside vendor known as a "caging agent" to handle any clerical tasks associated with the direct mail program, such as processing the responses, collecting and depositing the donations and updating the list based on mail returned as undeliverable.
Caging as applied pejoratively to a voter suppression tactic refers to times when a political party or other partisan organization sends registered mail to addresses of registered voters that they've identified as likely to be unfriendly to their candidate. All mail that is returned as undeliverable is placed on what is called a caging list. The group that sent the mail then systematically uses this list to challenge the registration or right to vote of those names on it, on the grounds that if the voters were unreachable at the address listed on his or her voter registraton, then their registration is fraudulent and they should not be allowed to vote.
The group that intends to challenge the validity of a voter based on this tactic might not do so until the day of the election. In this case, an agent of the party that is engaging in list caging would be present at the polling station and would challenge that voter with evidence of the returned mail, working from the caging list.
States where caging is illegal
Minnesota outlawed the practice of building voter caging lists compiled from returned mail sent by a political party after the 2004 election.
Examples of caging
On September 16, 2008, the Obama legal team announced that they would be seeking an injunction to stop an alleged caging scheme in Michigan wherein the state Republican party would use home foreclosure lists to challenge voters still using their foreclosed home as a primary address at the polls.  Although Michigan GOP officials called the suit "desperate", a judge found the practice to be against the law.,
A 2004 article in the Washington Post says, "In 1986, the RNC tried to have 31,000 voters, most of them black, removed from the rolls in Louisiana when a party mailer was returned. The consent decrees that resulted prohibited the party from engaging in anti-fraud initiatives that target minorities or conduct mail campaigns to "compile voter challenge lists."
New Jersey 1981
A 2004 article in the Washington Post says, "In 1981, the Republican National Committee sent letters to predominantly black neighborhoods in New Jersey, and when 45,000 letters were returned as undeliverable, the committee compiled a challenge list to remove those voters from the rolls. The RNC sent off-duty law enforcement officials to the polls and hung posters in heavily black neighborhoods warning that violating election laws is a crime."