The election is to decide who completes Lautenberg's term. Four Democrats and two Republicans have filed petitions to run in the Senate race, with three early polls showing Democratic Newark Mayor Cory Booker as the front-runner.
Christie scheduled the election for Oct. 16, 20 days before the general elections. A group of Democrats, backed by government watchdog groups and a coalition of minority political organizations, sued, saying it should be Nov. 5, the day voters are going to the polls anyway.
Christie's critics have complained that holding the election in October will cost taxpayers unnecessarily. Officials say each election costs the state about $12 million to run.
Judge Jane Grall wrote Thursday that objections to the costs of the election are policy matters that are not questions for the court.
"The question for us is whether the date selected is in violation of the law or constitution, and we conclude that it is not," she wrote.
The court ruled less than a week after the objection was filed and without taking oral arguments.
Somerset County Democratic Party Chairwoman Peg Schaffer, a lawyer for a group of Democrats that sued, said she would speak with other groups involved in the litigation to decide whether to appeal to the state Supreme Court. She said a decision could come Friday.
She noted that a key part of the legal interpretation came down to language in a state law saying the governor could call a special election for a date other than an already scheduled election day. She argued that a comma meant that that provision was only in certain cases; the state said it was in any case.
"It's a very expensive comma," Schaffer said. "I can't say they're wrong. I can say I disagree with them."
Complicated politics surround the dispute. Democrats said Christie did not want his reelection vote and legislative elections he is hoping his party wins on the same ballot as Booker, who like Christie is a nationally known figure. One of their fears is that voters will not turn out for one election in October and another in November.
"Regardless of the outcome of the court's decision, holding an election on a Wednesday in October is a cynical decision by Gov. Christie that will disenfranchise voters," said State Sen. Barbara Buono, the Democratic candidate for governor.
Some conservative Republicans say Christie should have appointed someone to serve the rest of Lautenberg's term, giving Republicans a better chance to hang on to the seat after the term expires in January 2015. The last time New Jersey elected a Republican to the Senate was in 1972.
Instead, Christie appointed state Attorney General Jeffrey S. Chiesa to the Senate, but only until a replacement can be elected. Chiesa, a Republican and confidant of Christie, is not seeking the seat beyond the short stay.
Christie has said he wants voters to be able to decide who represents them and for that to happen as soon as possible.