A federal judge has granted an emergency request to force New Jersey to redesign its election ballot before the June primary, upending a longstanding source of electoral power for the state’s Democratic and Republican parties.

The ruling, issued on Friday, is expected to fundamentally reshape politics in New Jersey and will have an immediate effect on June’s primary races.

“The integrity of the democratic process for a primary election is at stake,” wrote Zahid N. Quraishi of U.S. District Court for the District of New Jersey in a 49-page decision.

“Plaintiffs have put forth credible evidence not only that their constitutional rights are violated by the present ballot design used in New Jersey, which is used in no other state in the country,” he wrote, adding, “but that defendants would suffer minimal harm in implementing the ballot design requested.”

The implications of Judge Quraishi’s decision have loomed over a high-stakes race to replace Senator Robert Menendez, a New Jersey Democrat accused of accepting bribes in exchange for political favors.

Representative Andy Kim, a Democrat running for Mr. Menendez’s seat, had made concerns over the ballot’s fairness a defining theme of the race, and last month he filed a lawsuit that led to Friday’s judicial decision.