Elected representatives' interest in preserving relationships with their constituents merited permission to intervene in a redistricting action, as the interest was distinct from the interests represented by the other plaintiffs. In appellate review of the decision to deny permissive intervention, the equitable factors balanced are those prevailing at the time of the original decision to deny intervention.
http://www.opn.ca6.uscourts.gov/opinions.pdf/18a0194p-06.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.