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 New Mexico, a person or entity not originally part of a lawsuit can intervene in the case by filing a plea in intervention before the court's judgment is entered. This process does not require the court's permission, and it is the responsibility of the party opposing the intervention to challenge it by filing a motion to strike. The right to intervene is granted if the potential intervenor has a significant interest in the case, such that they could have initiated the same lawsuit or defended against it. The court should not strike an intervention if the intervenor passes the test of having a sufficient interest in the case, the intervention does not overly complicate the case with additional issues, and the intervention is nearly essential to protect the intervenor's interests. If these conditions are met and the court strikes the intervention, it may be considered an abuse of discretion. It's important to note that while these general principles apply, specific cases may be subject to additional rules or interpretations by the courts.