A person or entity who is not a party to a lawsuit generally may intervene in the lawsuit and become a party to it by filing a plea in intervention prior to the entry of the court’s judgment—provided the court does not reject (strike) the intervention. An intervenor is not required to secure the court’s permission to intervene, and the party who opposed the intervention has the burden to challenge it by filing a motion to strike.

A person or entity has a right to intervene if the intervenor could have brought the same lawsuit, or any part of it in his own name—or, if the action had been brought against him, he would be able to defeat the lawsuit, or some part of it.

A trial court abuses its discretion by striking an intervention if (1) the intervenor meets the above test, (2) the intervention will not complicate the case by an excessive multiplication of the issues, and (3) the intervention is almost essential to effectively protect the intervenor’s interest.

In Maine, the rules regarding intervention in a lawsuit are governed by the Maine Rules of Civil Procedure, specifically Rule 24. According to this rule, a person or entity may intervene in an ongoing lawsuit if they have an interest relating to the property or transaction that is the subject of the action. The intervenor must claim an interest that might be impaired by the disposition of the lawsuit and that interest may not be adequately represented by the existing parties. The intervention can be as of right or permissive. An intervention of right is allowed when the intervenor's interest is such that they will either gain or lose by the direct legal operation and effect of the judgment. Permissive intervention is at the discretion of the court and may be granted when an applicant's claim or defense shares a common question of law or fact with the main action. The burden to challenge an intervention lies with the party opposing it, and they must file a motion to strike the intervention. A trial court may be found to have abused its discretion if it strikes an intervention that meets the criteria of having a significant protectable interest, not complicating the case excessively, and being essential to protect the intervenor’s interests.