“It’s dark money,” Laura Fine, an Illinois state senator, said last week, struggling to explain how she had received hundreds of thousands in donations from super PACs tied to AIPAC. “Our campaign does not coordinate.”

One of her opponents, Mayor Daniel Biss of Evanston, Ill., shot back, “Your campaign is bankrolled by AIPAC and MAGA donors.” Then Kat Abughazaleh, a third candidate running to the left of both Mr. Biss and Ms. Fine, jumped in to attack them for what she called “the lying, the bickering over who likes AIPAC more.”

AIPAC, the hard-line pro-Israel lobbying organization that once commanded bipartisan fealty, has increasingly become a boogeyman in Democratic circles, with scores of candidates distancing themselves from the group. Last month, Gov. Gavin Newsom of California, a likely 2028 presidential candidate, said he “never will” accept money from AIPAC.

Nowhere is the divide sharper than in the Ninth District, a crooked finger that stretches from the Chicago lakefront through suburbs north and northwest of the city, a heavily Democratic and highly educated area with many historically Jewish communities. While AIPAC has rarely been involved in a race with dueling Jewish candidates, this one, with Ms. Fine and Mr. Biss, is an exception.

And the differences between them, on AIPAC and on Israel, mirror some of the divisions tearing through the wider Jewish community.

Neither AIPAC nor its official super PAC, United Democracy Project, is officially involved in the district, which has been represented by a Jewish Democrat for 61 years, either Sidney R. Yates, starting in 1965, or the now-retiring Representative Jan Schakowsky, elected in 1998.

In the Chicago area, Elect Chicago Women and Affordable Chicago Now — appeared suddenly, spending at least $10.9 million in the Ninth District primary and two other Illinois districts with competitive Democratic primaries.

Both new PACs have ties to groups that work closely with AIPAC. The Biss campaign and J Street, a liberal Jewish organization that is more critical of Israeli leadership under Prime Minister Benjamin Netanyahu, have called them AIPAC front groups.

Whether the deluge of money funneled from the groups will help the candidates favored by pro-Israel donors, including Ms. Fine, or backfire and drive voters critical of Israel toward one of her opponents, is an open question.

“In the Ninth District, AIPAC is toxic,” State Senator Mike Simmons, a candidate in the race who is critical of Israel, said in an interview.

On Tuesday, Democratic primary voters nominated a fierce critic of Israel, the Rev. Frederick Douglass Haynes III, for a heavily Democratic House seat in Dallas. And Mr. Newsom said this week that the United States should reconsider military aid to Israel, which he compared to an apartheid state.

Illinois’ governor, the billionaire JB Pritzker, was once a major donor to AIPAC, but he said in an interview last week that he “walked away” from the group around 2015, when it began to veer to the right.

“I still believe it is significantly MAGA-influenced,” said Mr. Pritzker, a Democrat.

Last month in New Jersey, AIPAC poured funds into campaign ads attacking Tom Malinowski, a moderate Democratic House candidate who supported Israel but said that U.S. aid should not be unconditional. Instead of helping a more pro-Israel opponent, Tahesha Way, the attacks turned voters from Mr. Malinowski to a pro-Palestinian progressive, Analilia Mejia, who won the race.

In Chicago, the group has focused much of its ire on Mr. Biss, a grandson of Holocaust survivors who spent many of his childhood summers in Israel, where his mother was born, and whose nuanced views on the Middle East reflect those of many liberal Jews, said Jeremy Ben-Ami, the executive director of J Street, which is backing Mr. Biss with $100,000.

“This is a guy who can’t possibly be considered anti-Israel — he is the quintessential American Jew,” Mr. Ben-Ami said. “He is at the 50-yard line of Jewish Americans, and AIPAC doesn’t want them anywhere near policy.”

Exactly who is funding the attack ads in and around Chicago will not be known until well after the primaries, when the groups finally have to disclose their donors.

“We don’t want somebody writing a blank check to Netanyahu,” said Patrick Clear, an Evanston resident who voted for Mr. Biss.

Mr. Clear and his wife, Barbara, said they were particularly turned off by the television ads promoting Ms. Fine, paid for by the Elect Chicago Women PAC, whose ties to AIPAC have been widely discussed in local political circles.

“It’s such an anodyne name, and then you see it’s entirely fueled by AIPAC,” Mr. Clear said.

Elect Chicago Women has so far spent more than $5.1 million to back Ms. Fine and $3.4 million to support Melissa Bean, a former congresswoman now running in the Eighth Congressional District. Affordable Chicago Now has spent at least $2.4 million to support Donna Miller, a Cook County Commissioner running in Illinois’ Second District.

United Democracy Project, the super PAC officially aligned with AIPAC, has spent more than $3.1 million in the Seventh Congressional District to support Melissa Conyears-Ervin, the Chicago city treasurer.

In an interview, Ms. Fine said she was “very surprised that there has been so much focus on this.” She said that she has repeatedly asked the groups backing her to reveal their donors, but she is “kind of hitting a wall” because they have not responded.

“I think this is a big problem in our political system,” she said. “It does a disservice to the electorate.”

Mr. Biss said that AIPAC sees him as a threat to their “far right, militaristic approach.”

“I think that it is really important to be willing to stand clearly in support of my values, which are aligned with the broad mainstream of American Jews,” he said, “and against a really hard-line point of view that is inaccurately trying to represent itself as speaking more broadly for the Jewish community.”