Voter 'caging' is a practice that is made possible by US election rules, and thus applies mostly to elections in that country. Other forms of skullduggery such as voter intimidation know no borders, but caging seems to be a uniquely American phenomenon.
What is Caging?
Caging is the practice of challenging the voting rights of a large number of people at the polls on election day. Someone compiles a list of voters whose right to vote can be challenged, and then these voters may be unable to vote or may have their ballot suspended pending an investigation. Notionally, the reason for this is to prevent election fraud, that is, by ensuring that individuals who are not eligible to vote in a particular location cannot cast a ballot there. In most cases, however, the true purpose of this tactic is to focus on a group of voters who are likely to support a candidate or party whose candidacy and/or policies the list-compiler opposes, and to disenfranchise them, that is, to suppress their votes. Such groups are almost always minority or low-income groups, although in some cases military service personnel or other groups have also been targeted.
Those hoping to employ some variation of the practice may refer to it as 'ballot security' or 'vote integrity' or by other similar terms. These names are meant to give a veneer of respectability to the practice. If it involves mass challenge of eligibility, though, it is caging, and is unethical at least, and in some circumstances it is either illegal or heavily constrained by court orders.
How does Caging work?
In the original implementation, non-forwardable mail was mass-mailed to a target community, often a particular ethnic group. The mail purported to confirm eligibility to vote. A list was compiled of all mail returned as undeliverable, and then on election day the ballots of anyone claiming to reside at those addresses were challenged. Court challenges have generally held that this method of challenging voting rights is impermissible.
In the modern era, widespread access to all kinds of electronic databases and forms of monitoring have created many more sophisticated ways of obtaining similar lists.
The ballot challenge process differs from state to state. Some states have antiquated laws which were actually intended to suppress the votes of African-American voters, and have yet to be updated. Other states are updating or have updated their laws to make caging less effective as a partisan tool. The state of Minnesota has outlawed caging entirely.
Who uses Caging?
Any partisan group could use this tactic, and there have been instances of its use from various points on the political spectrum. Republicans do seem to have a particular affinity for it, and the Republican National Congress is specifically constrained from using it by court order. The RNC has repeatedly tried to vacate the court's decree, to no avail at time of writing. State parties are under no such restriction unless their own state has made laws against it.
Web sites such as WikiLeaks have caches of emails that appear to implicate Republican officials in various levels of caging schemes.
Why does it matter?
Simply put, the primary purpose of caging is to deny your political opponent some meaningful number of potential votes, usually in the thousands, in hopes of swinging the results of a particular district in your favor. In areas where the race is forecast as particularly close, caging alongside other tactics has the potential to influence results in a meaningful way.
Discussions of how the current US presidential election is being handled, particularly from the Trump/Pence campaign, have raised the spectre of both Caging and Voter Intimidation being used as tools in the imminent election.
References
Motivated by curiosity and also by IN9.