TRENTON, N.J. (AP) — New Jersey’s primary ballot design unconstitutionally favors candidates who have the support of political party leaders and should be scrapped, U.S. Rep. Andy Kim, who’s locked in a primary contest for U.S. Senate against the state’s first lady Tammy Murphy, said in a federal lawsuit filed Monday.

The suit seeks to upend New Jersey’s unique primary ballot system in which candidates backed by political party leaders appear grouped together on the ballot, with challengers in separate columns, and takes aim at a system widely considered to be a crucial advantage in primary contests.

It comes as Kim and Murphy are facing off in a primary to succeed Democratic Sen. Bob Menendez, who faces federal corruption charges and hasn’t announced his plans on reelection. Murphy, whose spouse is Democratic Gov. Phil Murphy, won the backing of county party leaders in the state’s biggest counties soon after she announced her candidacy. She has said she worked for those endorsements herself and didn’t seek her husband’s support.

Kim’s suit against county clerks who design the ballots seeks to implement the kind of ballot that much of the rest of the country uses, listing all candidates next to the office they’re seeking.

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“When the choices of primary voters, who by law are the sole judges to determine a party’s nominee for the general election, are cynically manipulated by the Defendants, the result is anathema to fair elections,” the lawsuit says.

The issue has increasingly become a flashpoint, particularly among progressives, who have advocated for abolishing the so-called county line system.

Murphy spokesperson Alexandra Altman criticized Kim and called the lawsuit a “hypocritical stunt” aimed at furthering his career.

“Andy Kim doesn’t have a problem with the county line system, he has a problem with the idea of losing county lines — as he is perfectly happy to participate in the process when he wins,” Altman said in a text message.

A message seeking comment was left with the organization that represents county clerks.