The discovery of as many as 6,600 noncitizens on New Jersey’s voter rolls has thrust the state’s Democratic governor, Mikie Sherrill, onto a political tightrope.

And her balancing act is about to intensify.

Ms. Sherrill, who took over in January, on Tuesday defiantly refused the Justice Department’s demand that the state turn over personal details about the thousands of residents affected by what she has called a software error. “The request had no legal basis,” she said in response to the Trump administration’s request for the “full names, dates of birth, nationalities, residential addresses” of noncitizens who were inadvertently registered to vote between June 2023 and June 2024.

Ms. Sherrill has instead stressed that the noncitizens placed on the voter rolls when seeking driver’s licenses or IDs were not at fault and has vowed to do what she can to protect them.

She has chastised President Trump, accusing him of chasing ghosts of elections past and weaponizing elections “for political gain.” At the same time, the governor, a moderate in her party, signaled last week that she was open to “suggestions that make our elections more safe.”

She has also made it clear that the mistake did not happen on her watch. Ms. Sherrill has cast blame on the technology vendor that supplied the software, the Motor Vehicle Commission and the administration of her predecessor, Philip D. Murphy, a fellow Democrat.

“Unacceptable,” she said in a news conference — five times.

She merely inherited this mess, but cleaning it up could be complicated — both practically and politically.

About 400 of the wrongly registered noncitizens have cast ballots since June 2023, Ms. Sherrill said. The governor has said that she had no evidence, “at this time,” that any election was swayed, but has ordered an independent investigation. (In recent years, no statewide or New Jersey federal election has been close enough to have been decided by 400 votes.)

Nevertheless, the New Jersey situation has given Mr. Trump and his fellow election truthers an example of unlawful voting that everyone agrees happened and is a problem.

“The president has made it a national issue,” said Benjamin Ginsberg, a prominent Republican election lawyer, who has been critical of the president’s unsubstantiated claims of stolen elections.

“And Governor Sherrill,” he continued, “whether she likes it or not, is now the face of that issue.”

Ms. Sherrill served in Congress before being elected governor by a 14-point margin last year. She has established a federal political action committee to support “cleareyed leaders who are unafraid to challenge the status quo,” and has said that she expects the next president to be from the Democratic Party.

“I want to be influencing that ticket very heavily,” Ms. Sherrill said in a July 20 interview about what she considered her state’s successful handling of its World Cup hosting duties.

A day later, she held a news conference and disclosed that a flaw in software used at motor vehicle offices between June 12, 2023, and June 25, 2024, had resulted in thousands of noncitizens being registered to vote, even though they had marked that they were not U.S. citizens and were therefore ineligible.

The software was fixed, Ms. Sherrill said, but the Murphy administration never notified affected residents or removed their names from the rolls. Mr. Murphy, through a spokesman, has said that he was unaware of the breach.

Knowingly registering to vote or casting a ballot as a noncitizen is illegal. And as many as 6,600 people now stand in harm’s way as the Trump administration intensifies its immigration crackdown.

Since last year, when the Justice Department began demanding voter rolls nationwide, more than half of the states, including New Jersey, have refused, leading to lawsuits. Judges have repeatedly ruled against the Trump administration, finding that states were not obligated to release the data to the federal government.

Ms. Sherrill said Tuesday that the state was moving to quickly delete about 4,500 of the affected residents from the rolls.

The remaining 2,100 registrations are being referred to county election officials. “We are taking this extra step because our initial review indicates that a number of these individuals may be citizens,” she said in a statement. “Just as we have an obligation to remove everyone who is not eligible to vote, we also have a responsibility to protect the voting rights of eligible New Jersey citizens and ensure no one is improperly removed from the rolls.”

Last week, the Motor Vehicle Commission began mailing letters, in English and in Spanish, alerting those affected. “Notwithstanding your selection of ‘No’ to the U.S. citizenship question, your information was transmitted to the Statewide Voter Registration System for voter registration,” the letters read. The state also set up an information portal, “MVC System Error 2023.”

Fear in immigrant communities, already high during Mr. Trump’s second term, has soared, said Maria Juega, a founder and former executive director of the Latin American Legal Defense and Education Fund, an immigrant rights group in Trenton, the state’s capital.

“The atmosphere couldn’t be more tense,” Ms. Juega said. “People are scared to death of leaving their houses.”

New Jersey is one of 19 states that both allow undocumented immigrants to obtain driver’s licenses and make it easy to register to vote at motor vehicle offices. A website where users can look up whether they are registered to vote is cranky and in English, Ms. Juega noted, fueling confusion among immigrants who now fear they may unwittingly be at risk.

“It’s just a mess — a total mess,” she said.

Ms. Sherrill has promised transparency and committed to holding those responsible accountable.

“We inherited a problem. We didn’t hide it. We didn’t deny it,” she said Tuesday.

But the timing of the discovery could hardly be worse, with less than 100 days until the midterm elections.

“She doesn’t have the luxury of time,” said Micah Rasmussen, director of the Rebovich Institute for New Jersey Politics at Rider University.

It remains an open question how much the controversy will resonate beyond New Jersey. But Republicans in Trenton and in Washington have said that Ms. Sherrill’s effort to make clear that the error took place on Mr. Murphy’s watch is a distinction without a difference.

Joe Pennacchio, a Republican state senator, said Democrats were to blame. “They were in charge of the hen house,” he said.

The Democratic response in New Jersey has been notably muted.

State Senator Andrew Zwicker was one of the few Democrats who offered a full-throated defense of New Jersey’s elections after Ms. Sherrill’s disclosure, stressing how small a fraction of the total vote was involved.

Still, Mr. Zwicker acknowledged that New Jersey’s voter-registration error has “been sucked up into” Mr. Trump’s narrative of widespread illegal voting.

“It’s utterly discouraging,” he said.

The discovery coincides with Mr. Trump’s efforts to persuade Congress to adopt his SAVE America Act as he pushes for more control over voting ahead of November’s elections. On Monday, the Trump administration asked the Supreme Court to clear the way for the president to restrict mail voting by executive order.

Mr. Ginsberg, the election lawyer, said it was possible for Ms. Sherrill to turn the controversy to her advantage, if, for example, she were able to create a bipartisan model for auditing voter rolls that other states could emulate.

“It’s an interesting test for somebody who wants to be on the national stage,” he said. “There’s a chance for her to turn something really messy into an opportunity.”