New Jersey moved a step closer last week toward overhauling its unique-in-the-nation election ballots, in a decision that could reshape party politics in the state for years to come.

But not — at least not immediately — for both major parties.

On Saturday, the federal judge who ordered the redesign, in response to a lawsuit filed in February by three Democratic candidates, said in a statement that only the Democratic primary, which includes the race to replace Senator Robert Menendez, would have to use the new ballot. The Republican ballot, he wrote, can stay the same, though he said his order did not prohibit Republican leaders from choosing to alter their party’s ballot.

The clarification is the latest twist in a long legal battle in New Jersey to shift the balance of electoral power away from party-backed candidates and open the door for newcomers in both parties. But if the decision stands, Republicans, too, may soon be forced to change their ballot, though perhaps not in time for the June 4 primary, said Julia Sass Rubin, a professor of public policy at Rutgers University who was an expert witness in the lawsuit.

“It’s just a hiccup,” Dr. Rubin said. “If this decision holds, it will completely upend New Jersey politics.”

On Friday, the federal judge, Zahid N. Quraishi of U.S. District Court for the District of New Jersey, ruled in favor of changing the format of primary election ballots used in 19 of 21 counties in New Jersey, which have historically favored candidates put forward by party bosses.

The so-called county-line ballot, in which local political leaders’ preferred candidates are grouped together in a prominent position, is an anomaly in the United States, with only New Jersey using the system, said Nicholas Stephanopoulos, a professor at Harvard Law School.