The shift has put Sacks, 64, at the center of disputes over city taxes, public education and the influence of wealthy donors. He is supporting Secretary of State Alexi Giannoulias, a challenger to Johnson, and backing Common Ground Collective, a political action committee that has opposed parts of Johnson’s budget and is supporting candidates in the Nov. 3 school board election who oppose the Chicago Teachers Union’s preferred slate.

The conflict has made Sacks’ long record in Chicago politics newly visible. A major Democratic donor and the longtime head of investment firm Grosvenor, he has advised elected officials, raised money for candidates and backed causes ranging from gun violence prevention to reproductive health. He also helped recruit J.B. Pritzker to run for governor and led fundraising for Chicago’s 2024 Democratic National Convention.

His political spending has grown more direct. Sacks and his wife have spent $9.2 million on Illinois Democratic candidates and causes since the start of 2025, according to campaign records cited in the reporting. Sacks has also given about $500,000 to Giannoulias’ mayoral campaign this election cycle.

Johnson and Sacks have offered sharply different explanations for their falling-out. Johnson has blamed Sacks for helping defeat the mayor’s proposed corporate head tax, a claim Sacks denies. The mayor has also connected the political opposition he faces to his 2024 vote for a City Council resolution calling for a ceasefire in Gaza. Sacks has said he did not participate in a separate dispute over a proposed loan involving Chicago Public Schools.

A Johnson spokesperson said the mayor has had to respond to Sacks and Common Ground Collective, which the spokesperson said has targeted the city’s progressive movement. Sacks declined an on-the-record interview but provided selected responses to questions.

The Chicago Teachers Union has been among Sacks’ fiercest critics. Jackson Potter, the union’s vice president, points to Sacks’ close ties to former Mayor Rahm Emanuel, whose administration closed 50 public schools in 2013, most of them in Black neighborhoods. Potter also objects to Sacks’ role in backing school board candidates opposed by the union and says major political decisions about CPS should be shaped by families and residents who rely on the district.

Sacks has defended his involvement as a commitment to public education, saying his mother taught in CPS. His supporters cite other parts of his record: under Emanuel, he led the city’s public-private economic development agency and helped promote the Neighborhood Opportunity Fund, which directed some downtown development revenue to commercial projects on the South and West sides. Emanuel also credited Sacks with helping secure a state school funding formula that brought additional money to CPS.

That funding fight also exposed a lasting disagreement with the union. Sacks says the CTU did too little to help win votes for the formula. The union, meanwhile, opposed the accompanying Invest in Kids program, which offered tax credits to donors supporting private-school scholarships. Critics argued the program diverted support from public schools.

Sacks’ relationship with Johnson began on friendlier terms. Four years ago, Sacks contributed $200,000 to Johnson’s inaugural committee and met with the mayor-elect shortly after his runoff victory to discuss setting up the administration. The relationship soured amid disputes involving the City Council, CPS and the union. Sacks says he told CTU President Stacy Davis Gates in 2024 that he would support Common Ground Collective and was prepared to spend heavily in response to attacks on him.

The political fight has also drawn accusations of antisemitism, which Sacks has said he has faced. Potter, who is Jewish, rejects that characterization of criticism from the union and says the focus is Sacks’ spending and political influence. State Sen. Robert Peters, also Jewish, has described scrutiny of Sacks as legitimate given his public role, while warning that treating every criticism as antisemitic can dilute the meaning of actual antisemitism.

The dispute reaches beyond personal animosity. Johnson’s allies argue that Sacks should answer for his influence during Emanuel’s administration, including the school closures. Lisa Schrader, Emanuel’s former chief of staff and a current Sacks employee, has countered that no single adviser was involved in every decision. Labor leader Bob Reiter, meanwhile, says Sacks has supported organized labor and helped mediate disputes, including a settlement that ended a downtown hotel workers’ strike.

As school board and mayoral campaigns approach, Sacks’ spending and alliances are likely to remain a central issue. The contests will test competing claims about whether his involvement represents civic investment and political participation — or outsized private influence over decisions affecting Chicago’s public schools and neighborhoods.