The New Jersey Legislature, the Constitutionally-authorized state authority regarding the election of Senators, did NOT authorize the NJ judiciary to make this kind of exception. The sole role they gave the NJ judiciary with regard to ballot vacancy was to determine whether a candidate was qualified or not qualified in accordance with the law.
Had the statute regarding ballot vacancies read something like "...no less than 51 days prior to the election except in cases that the judiciary may see fit....", AND there was no federal law contradicting this with regard to Senate elections (Congress does have supremacy in this regard), then you would have a strong states' rights case.
Oh, don't get me wrong. I think this is judicial activism at its worst - a terrible decision on the part of the NJ Supremes. But I also don't see what authority the SCOTUS has to hear a case regarding internal state politics. If SCOTUS were to overturn NJ's ruling they would need to employ judicial activism to reverse...judicial activism.
Mr. Egg...wrong. The Judicial Review of legislative statute is now part of the structure of the government. There is even a precedent on this law anyway where a candidate died, and was replaced within the 51 day window. So there are uncontested exceptions, and if the legislature wants to weigh in for next time, they will. (probably won't) So goes liberal interpretations by courts.