Chicago Police Board President Lori Lightfoot resigned Monday from the nine-member board that will decide the fate of Chicago Police Officer Robert Rialmo, setting the stage for Lightfoot to challenge the mayor who appointed and re-appointed her.

Lightfoot, 55, could not be reached for comment on her reasons for resigning or the timing of her decision. She has already hired a pollster, a political consultant, a fundraiser and a firm to handle her direct mail.

In her resignation letter, Lightfoot kept it short and sweet.

“I hereby resign from my roles as the President and member of the Chicago Police Board,” Lightfoot wrote to the mayor who appointed and re-appointed her.

“As I step away from the Police Board, I will continue my efforts to be a passionate advocate for issues that are critical to uplifting the quality of life for all Chicagoans. It has been my great honor to serve on the Board and to tackle the many issues that came before us during my tenure.”

Mayor Rahm Emanuel’s communications director issued an emailed statement in response to Lightfoot’s resignation that made no mention of the mayoral campaign ahead.

“The Mayor has placed a premium on ensuring we have a strong system of accountability to build trust with residents and support the city’s strong public safety strategy. He will select a new chair of the police board to carry forward those values,” Adam Collins wrote.

Lightfoot’s resignation means the Police Board will be without a president when it decides whether Rialmo should be fired for shooting a bat-wielding Quintonio LeGrier and LeGrier’s neighbor Bettie Jones in December 2015.

Lightfoot has been on a political collision course with Emanuel for three years — since the day he chose her to overhaul a Police Board with a history of reversing the superintendent’s recommendations to terminate accused officers.

She led the nationwide search for a replacement for fired Police Supt. Garry McCarthy, only to have the mayor reject all three finalists and choose Eddie Johnson, who hadn’t even applied. McCarthy is now a mayoral challenger.

Lightfoot also co-chaired the Task Force on Police Accountability, whose scathing report forced Emanuel to abolish the Independent Police Review Authority.

Last year, she branded a memorandum drafted by Emanuel in hopes of avoiding federal court oversight of the Chicago Police Department a “fundamentally flawed” document that “sets the Police Department up for failure.”

In March, Lightfoot told the City Club of Chicago that the Chicago Police Department “desperately needs” a new training academy, but Emanuel’s plan to build a $95 million complex in West Garfield Park is “ill-conceived.”

“Putting this edifice to policing in this high-crime, impoverished neighborhood where relations between the police and the community are fraught, without a clear plan for community engagement, is a mistake,” Lightfoot said.

“The allocation of any funds for a police academy … is viewed by many as further affirmation that needs of the people will never be prioritized over those of the police.”

The following day, Lightfoot warned Emanuel that he would face a furious political backlash if he refuses to empower a civilian oversight board to fire the police superintendent and establish police policy or if he tries to stall a City Council vote on the proposal until after the 2019 mayoral election.

Last year, Emanuel reappointed Lightfoot to another term as Police Board president, only because he was boxed in by the politics of police reform.

Ousting an outspoken African-American woman would have triggered a political backlash at a time when Emanuel is trying desperately to restore trust among black voters shattered by his handling of the Laquan McDonald shooting video.

“It would have sent a signal that he was unwilling to tolerate dissent and unwilling to implement reforms,” Ald. Howard Brookins (21st), former chairman of the City Council’s Black Caucus, said then.

“She has been very critical, which is why she has gained so much credibility from the activist community, despite the fact that she comes from the U.S. Attorney’s office” and once ran the Chicago Police Department’s Office of Professional Standards, a precursor to the Independent Police Review Authority, the alderman said.

The night before the reluctant reappointment, Emanuel and Lightfoot held a no-holds barred meeting in the mayor’s office, ostensibly to clear the air.

It was their first face-to-face meeting in 16 months. The mayor had two top aides in attendance, an apparent indication of the distrust between the two adversaries.

“We had a full and frank discussion — and it was a dialogue, not a monologue,” Lightfoot said on that day.

“The mayor is a very candid guy and I appreciate his candor. Hopefully, he appreciates my candor and my independence.”

At the time, Lightfoot refused to say whether Emanuel had questioned her about whether she had any intention of challenging him in the 2019 race for mayor. But she said then: “I would not be seeking re-appointment to this job if I was running for mayor.”

Obviously, that’s changed. Monday’s resignation is the first shoe to drop. The second shoe will come when Lightfoot declares her candidacy for mayor, joining a crowded field that already includes seven other challengers.