Who can, in the US, challenge the registration of a voter?

Question

According to this article, many (predominantly black) voters were purged of the right to vote after mail delivered to their adress bounced, with the NAACP suing against this :

The lawsuit charges that Beaufort, Moore, and Cumberland counties are challenging voters’ eligibility with no other evidence than a single piece of mail that was sent to their home and bounced back as undelivered. [...] Arthur, who is African American and lives in a nursing home, grew up in Beaufort County and has voted in at least 14 elections. He testified that he did not receive any notice from the state or the county that his voter registration had been challenged [...]

The confusing part is this:

Shane Hubers, a Republican who ran for local office last year, challenged the registration of dozens of voters — the majority of them Democrats — in Beaufort County. In Cumberland County, a single individual used returned mail to challenge the registrations of 3,951 voters. In Moore County, N. Carol Wheeldon, the secretary of the local Republican Party, challenged approximately 400 registered voters. [...] In Moore County, Hair added, “the return address of a right-wing group called the Voter Integrity Project appeared on the piece of mail that was used to challenge voters.”

It seems like a pretty clear cut case of disenfranchising voters because they would vote "wrong". What I don't understand - From the wording, it does not appear these people (and the 'Voter Integrity Project') sent the mails in any official capacity. So why does the body organizing the election react to anybody claiming their mail to certain voters bounced? Could in theory anybody challenge anybody's voter registration in North Carolina?

Answer 1

The National Voter Registration Act of 1993 requires that each state shall "conduct a general program that makes a reasonable effort to remove the names of ineligible voters from the official lists of eligible voters" by reason of the death of the registrant or a change in the residence of the registrant.

North Carolina General Statutes - Chapter 163 Article 8 specifies a "right to challenge":

Any registered voter of the county may challenge the right of any person to register, remain registered or vote in such county. No such challenge may be made after the twenty-fifth day before each primary, general, or special election.... Each challenge shall be made separately, in writing, under oath and on forms prescribed by the State Board of Elections, and shall specify the reasons why the challenged voter is not entitled to register, remain registered, or vote. When a challenge is made, the board of elections shall cause the word "challenged" to be written in pencil on the registration records of the voter challenged. The challenge shall be signed by the challenger and shall set forth the challenger's address

Nine reasons are specified as the grounds for a challenge:

• (1) That a person is not a resident of the State of North Carolina, or
• (2) That a person is not a resident of the county in which the person is registered, provided that no such challenge may be made if the person removed his residency and the period of removal has been less than 30 days, or
• (3) That a person is not a resident of the precinct in which the person is registered, provided that no such challenge may be made if the person removed his residency and the period of removal has been less than 30 days, or
• (4) That a person is not 18 years of age, or if the challenge is made within 60 days before a primary, that the person will not be 18 years of age by the nextgeneral election, or
• (5) That a person has been adjudged guilty of a felony and is ineligible to vote under G.S. 163-55(2), or
• (6), (7) Repealed by Session Laws 1985, c. 563, ss. 11.1, 11.2.
• (7a) That a person is dead, or
• (8) That a person is not a citizen of the United States, or
• (9) With respect to municipal registration only, that a person is not a resident of the municipality in which the person is registered, or
• (10) That the person is not who he or she represents himself or herself to be

Procedures are detailed for conducting a preliminary hearing and then a hearing on the challenge, specifying "The presentation of a letter mailed by returnable first-class mail to the voter at the address listed on the voter registration card and returned because the person does not live at the address shall constitute prima facie evidence that the person no longer resides in the precinct."

If the county board of elections finds probable cause for a challenge during the preliminary hearing, they are required to schedule a hearing on the challenge itself. Notifications are sent to the challenged registrant -- by first-class mail, at the address listed on their voter registration record -- and to the heads of each political party in the county. The challenged registrant is required to appear at the hearing with an affidavit that he or she is eligible to vote within the jurisdiction in question; a representative may appear in his or her place.

Answer 2

Depends by the state. I am linking a resource that shows what election precinct officials and poll watchers, which can be anyone, are allowed to do, on what grounds, and the evidentiary and process standards for so doing.