Plenary prevents the courts' or executive branches' influence on election laws. Elections are political, as opposed to lawful. One exception is the ability of a state governor to supply the slate of electoral college electors if the state legislatures is at an impasse to do so. Courts can only determine if election was UNLAWFUL, and turn it back over to the legislature to work out with Congress.
IANAL, I am not a lawyer. So the courts may exercise other powers to place conditions like successful audits in next election to see if the state keeps its Congressional representation.