Professor Volokh details the basis for a federal suit to overturn the NJ Supreme Court order allowing the Dems to switch candidates for the Senate. I think it's cogent.
After reading the text of the ruling, I have to wonder if this court even has to wait for a petition to change the ballots. It emphasizes that "the election
statutes should be liberally construed to allow the greatest scope for public participation in the electoral process, to allow candidates to get on the ballot, to
allow parties to put their candidates on the ballot, and most importantly, to allow the voters a choice on Election Day." (italics added) Apparently the court feels that the withdrawal of a candidate is such a serious problem that it must throw out statutory limits. If that is so, what is there to stop the court from determining on its own, that the candidates or either major party are insufficiently "competitive." Suppose nobody filed to run for the Senate seat on the Republican ticket, the court's holding that it "is in the public interest and the general intent of the election laws to preserve the two-party system and to submit to the electorate a ballot bearing the names of candidates of both major political parties" suggests that such a failure would justify the court in adding its own nominees.