As with most activities that can impact others, it is difficult to provide exact criteria for when the activity becomes criminal. It often depends on the exact circumstances. Opposition research, which can serve to uncover corruption or unethical or illegal behaviour by persons in the public sphere, may serve important an important role in a functioning democracy. Some criteria to distinguish harassment from legitimate opposition research may include:

1) Intent. If the intention of the actors is to uncover illegal or unethical behaviour in order to inform the public of such behaviour, even if it serves the actor's own political goals, it may be considered legitimate opposition research. If the intention is to intimidate or coerce someone with information or evidence so obtained, especially if done clandestinely, it may be considered harassment or another potentially criminal activity.

2) Means. If the activity involves legal and ethical means, such as searching publicly available records, obtaining information lawfully from other sources such as government entities or other involved persons, or publicly confronting someone with information so obtained, it may be considered legitimate oppositions research. If it involves means such as physically harming others, obtaining information unlawfully or committing crimes such as theft, trespassing or even assault, it would likely be considered harassment.

3) Context. It is very important to consider the circumstances. Targeting a person who is not connected directly or indirectly to the public sphere or who is not considered to have obligations to the public, such as private citizens as opposed to politicians or government officials, would be more likely considered harassment as opposed to opposition research. 

Overall, distinguishing harassment from opposition research is a difficult and nuanced question, depending on factors such as intent, means and context. To answer it conclusively may require a judge or jury to make a decision after carefully considering all relevant factors.
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