Minnesota
Related: About this forumShould anyone run into trouble at the polls on November 8 be advised that MN has some rules about
who can challenge a voter and how.
http://www.sos.state.mn.us/elections-voting/election-day-voting/rules-for-challengers/
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Minnesota law allows you to challenge another voters eligibility if you have personal knowledge that the voter isn't eligible to vote there.
Who can be a challenger?
An election judge
An eligible voter from the same precinct as the voter being challenged
An appointed challenger (see more below)
What is the challenger's role?
The only action a challenger may take is to contest a voters eligibility, if and only if they have personal knowledge of that voters ineligibility. Suspicion is not a basis for making a challenge. The challenger must personally know that a specific person is not eligible to vote for a specific reason.
see also: Minnesota Statutes 204C.07 and Minnesota Statutes 204C.12 address challenges to voters.
And:
https://www.revisor.mn.gov/statutes/?id=204c.06
204C.06 CONDUCT IN AND NEAR POLLING PLACES.
Subdivision 1.Persons allowed near polling place. An individual shall be allowed to go to and from the polling place for the purpose of voting without unlawful interference. No one except an election official or an individual who is waiting to register or to vote or an individual who is conducting exit polling shall stand within 100 feet of the building in which a polling place is located. "Exit polling" is defined as approaching voters in a predetermined pattern as they leave the polling place after they have voted and asking voters to fill out an anonymous, written questionnaire.
Subd. 2.Individuals allowed in polling place; identification. (a) Representatives of the secretary of state's office, the county auditor's office, and the municipal or school district clerk's office may be present at the polling place to observe election procedures. Except for these representatives, election judges, sergeants-at-arms, and challengers, an individual may remain inside the polling place during voting hours only while voting or registering to vote, providing proof of residence for an individual who is registering to vote, or assisting a disabled voter or a voter who is unable to read English. During voting hours no one except individuals receiving, marking, or depositing ballots shall approach within six feet of a voting booth, ballot counter, or electronic voting equipment, unless lawfully authorized to do so by an election judge or the individual is an election judge monitoring the operation of the ballot counter or electronic voting equipment.
(b) Teachers and elementary or secondary school students participating in an educational activity authorized by section 204B.27, subdivision 7, may be present at the polling place during voting hours.
(c) Each official on duty in the polling place must wear an identification badge that shows their role in the election process. The badge must not show their party affiliation......
....Subd. 6.Peace officers. Except when summoned by an election judge to restore the peace or when voting or registering to vote, no peace officer shall enter or remain in a polling place or stand within 50 feet of the entrance of a polling place.
There is more detail at both links.