In Pennsylvania, any candidate in an election may appoint up to two people per election district to be poll watchers, provided they are eligible voters in the county where they'd be watching. These poll watchers, as well as any other eligible voter or election officer, are able to challenge a voters 'qualifications', essentially a voter's proof of identity and residency, if they believe the voter is not being honest about their identity/residency. If an elections officer believes the challenge is legitimate, then voter has two options: have another qualified voter sign an affidavit attesting to their qualifications and cast a vote like normal, or they can sign an affidavit themselves and cast a provisional ballot.
This provisional ballot will be examined by the county board of elections within 7 days of the election. In addition to poll watchers on election day, candidates and political parties can each select one representative to oversee the counting of provisional ballots, and they are allowed to challenge the voter's qualifications. If challenged, the county board of elections schedules a hearing within 7 days of the challenge, allowing the voter to attest that they are who they say they are and live where they say they live, and the county board of elections will then decide whether to count their vote. Any person 'aggrieved' by this decision can appeal it, and within two days this appeal is sent to a trial court where the decision could get reversed.
In short, a poll watcher (or any other eligible voter) can challenge a person's eligibility to vote if they believe the voter is lying. If they can prove and if challenged the voter can either get someone else to vouch for them or cast a provisional ballot. This provisional ballot is counted within a week after election day, and if the provisional ballot is also challenged then voter will be called to a hearing to attest to their eligibility to vote.
The Pennsylvania Department of State site has a page with election-related documents. One doc, Rights of Watchers, Candidates, and Attorneys, contains a list of rights that poll watchers have, with citations to the relevant law:
Section 417 of the Code, 25 P.S. § 2687, outlines the rights of duly appointed poll
- Watchers allowed in the polling place are permitted to keep a list of voters.
- Watchers allowed in the polling place are entitled to challenge the qualifications
of voters in accordance with the provisions of section 1210(d) of the Code (25
P.S. § 3050(d)). Section 417(b)
- Watchers allowed in the polling place are entitled to inspect the voting check list
and either of the two numbered lists of voters during those intervals when voters
are not present in the polling place, provided that the watcher does not mark
upon or alter any of these official records. (The judge of election must either
personally supervise or delegate supervision of such inspection of the list or
lists.) Section 417(b)
The first and third points are pretty straightforward: poll watchers can keep a list of people who voted, and can take a supervised look at the official records of voters.
The second point covers a poll watcher's ability to challenge the qualifications of a voter, and this is covered in more detail in the Pennsylvania statute 25 P.S. § 3050. This statute contains pretty much everything I said in my summary at the start of this answer, but the relevant part for what poll watchers can do is in section 4:
...any person, although personally registered as an elector, may be challenged by any qualified elector, election officer, overseer, or watcher at any primary or election as to his identity, as to his continued residence in the election district or as to any alleged violation of the provisions of section 1210 of this act, and if challenged as to identity or residence, he shall produce at least one qualified elector of the election district as a witness, who shall make affidavit of his identity or continued residence in the election district...
The Pennsylvania Department of State site also has the following information about provisional ballots:
Sometimes the County Elections Office needs more time to determine a voter's eligibility to vote. We may ask that voter to vote with a provisional ballot. A provisional ballot records your vote, while the County Election Office determines whether it can be counted.
You have the right to vote with a provisional ballot if:
- Someone challenges your eligibility to vote. You may produce a witness to sign an affidavit to affirm your identity and residency. If you produce a witness, you can vote either by paper ballot or on the machines. If you are unable to or choose not to produce a witness, you can vote with a provisional ballot.
The process for how provisional ballots are counted is covered in Section A of the statute quoted earlier (25 P.S. § 3050). It is a bit too convoluted to easily quote, but my summary at the start of this answer covers the important parts.
Some clarifications and extra info in response to comments:
In order to challenge someone, the poll watcher must prove to the election officer's 'satisfaction' that the voter has violated some voter qualification. From statute 25 P.S. § 3051:
(b) The vote of any elector shall be rejected by the election officers if they or any one of them shall of their own knowledge know him to be guilty of a violation of any of the provisions of this section, or if upon challenge of such elector by any qualified elector, election officer, overseer or watcher, it shall be proved to their satisfaction that such elector has violated the provisions of this section, and in no case shall any elector so challenged be permitted to vote, unless he shall make written affidavit that the matter of the challenge is untrue.
So, if a poll watcher is randomly accusing voters of not being eligible and offering no reason for the accusation (or at least one that an election officer believes), the challenges will likely not be upheld by the election officers, and the voters will be able to cast regular ballots.
If a challenger causes enough of a disruption to prevent the election officers from holding the election or intimidates voters in a way that influences them 'unduly', then they would likely be breaking the law per 25 P.S. § 3527. However, Law SE would be a better place to answer what constitutes an illegal disruption of the election, and when these challengers can be removed from the polling place by police.