Saying “this has been the job of a lifetime, but it is not a job for a lifetime,” Mayor Rahm Emanuel announced Tuesday morning he will not seek re-election to a third term.

Having just returned from dropping off his youngest child at college — she’s a freshman at Princeton — Emanuel and his wife, Amy Rule, apparently returned home with his mind made up.

“Amy and I have decided it is time to write another chapter together,” added Emanuel, who told staffers of his decision at a private meeting just before his public announcement at City Hall. “Amy and I are still young — and Amy still looks it.”

Married 27 years, the two are now officially empty-nesters, he said.

“What matters most is four more years for our children, not four more years for me,” he added. When they first got married, he said, he had told her he would never run for office — “and I’m about six elections behind the eight-ball.”

Emanuel, a former congressman who also held positions in two Democratic presidential administrations, was elected in 2011 and survived a run-off to win a second term in 2015. But he faced in increasingly crowded field for next year’s municipal election.

He also was looking at some turbulent weeks ahead, during the trial of Chicago Police Officer Jason Van Dyke, charged with murder in the October 2014 shooting death of Laquan McDonald.

In an event that has come to define his second term, Van Dyke was caught on video fatally shooting McDonald 16 times — but the video was not released for more than a year.

Van Dyke’s trial, which starts this week, is certain to dredge up ugly memories of Emanuel’s decision to withhold the McDonald shooting video until after the 2015 election and release it only after a judge ordered the city to do so.

That release sparked street protests and, eventually, a Department of Justice investigation into the Chicago Police Department. The DOJ probe produced a scathing report that faulted the department for what it called widespread constitutional abuses.

The report also offered stark anecdotes of deadly force, including officers firing shots in residential neighborhoods, and “dangerous practices” like taking people into gang-rival territories to “display” them.

Emanuel and Illinois Attorney General Lisa Madigan are nearing the end of negotiations over a court-enforced consent decree outlining department reforms recommended by that report.

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Kofi Ademola, with the Chicago chapter of Black Lives Matter, called Emanuel’s announcement a “great victory” for grassroots organizers who, in the wake of the McDonald video’s release, had worked to defeat Cook County State’s Attorney Anita Alvarez (who lost a re-election bid in 2016) and also demanded the firing of CPD Supt. Garry McCarthy.

Emanuel did fire McCarthy, in December 2015, and McCarthy now is running for mayor.

Ademola said the closure of a record 50 schools, mental health clinics and the lack of police accountability and an elected school board brought people from all over the city together against the mayor. Now, he added, the hope is to get “somebody in there who will be progressive. We have a lot of educating to do with the public on who is in the field.”

Activists who had repeatedly called for Emanuel’s resignation planned a celebratory rally for Tuesday afternoon on Daley Plaza.

Another challenge that had been on Emanuel’s radar was a term-limit measure being pushed by former Gov. Pat Quinn.

Quinn had submitted petitions for a ballot measure that that limit Chicago mayors to two terms. But he said his efforts to get such a referendum on the ballot in November weren’t aimed personally at Emanuel — and he said Tuesday he won’t run for mayor.

But if Quinn still isn’t interested, many others are. Besides McCarthy, the field now includes former Chicago Public Schools CEO Paul Vallas, Circuit Court Clerk Dorothy Brown, former Police Board President Lori Lightfoot, community activist Ja’Mal Green, former CPS principal Troy LaRaviere, businessman Willie Wilson and tech entrepreneur Neal Sales-Griffin.

Another oft-cited potential challenger, Cook County Board President Toni Preckwinkle, had taken herself out of the race. Her initial statement issued in reaction to Emanuel’s announcement did not hint at any change in those plans.

“I was surprised by Mayor Emanuel’s announcement this morning that he has decided not to seek re-election. I want to thank him for leading our city for the past eight years and also for his service to the nation – his time in Congress and as Chief of Staff to President Obama. Being mayor of the nation’s third-largest city is extraordinarily difficult and all-consuming. I wish him and Amy well in their future endeavors.”

Others also sang his praises.

“Chicago is better and stronger for his leadership, and I was a better president for his wise counsel at a particularly perilous time for our country,” former President Barack Obama said in response to the announcement. Emanuel had been his White House chief of staff.

“I’ve been blessed to call Rahm my friend. Whatever he chooses to do next, I know he’ll continue to make a positive difference, just as he has throughout his career in public service.”

Emanuel also served in the White House under President Bill Clinton.

“From the earliest days of my presidential campaign over a quarter-century ago, through my time in the White House; his service in Congress; as Chief of Staff to President Obama; and for eight vital years as Mayor, Rahm Emanuel has served with vision, purpose, principle, and impact,” Clinton said in a statement.

“I believe he succeeded because he cares about people, policy, and politics. Even people who disagree with him strongly on some issues understand that.”

Not everyone was sending Emanuel off with a pat on the back, however.

Emanuel had famously tussled with Karen Lewis, then the president of the Chicago Teachers Union, during a 2012 strike. That battle raised Lewis’ stature to the point that she pondered her own run for mayor — until a brain tumor put her on the sidelines.

Lewis’ successor, Jesse Sharkey, declared victory with Emanuel’s departure.

“Rahm came in like a wrecking ball, but he couldn’t wreck our love for the schools,” Sharkey said at a news conference Tuesday afternoon.

“He couldn’t make us budge from our defense for the schools and the city. This union, the CTU, followed Karen Lewis into battle, and today we’ve won,” Sharkey said.

“Maybe it’s different down in Emerald City, in the gleaming downtown, but out in the neighborhoods this is not a popular mayor. In the working class parts of this city, this is not a popular mayor.”

Just last week, Emanuel, along with other candidates for mayor, had begun circulating nominating petitions to gather the 12,500 signatures he would have needed to get on the ballot.

Emanuel had 1,400 volunteers — including Democratic ward committeemen, labor and community leaders — passing his petitions.

But even though he had been raising money, gathering signatures and putting a campaign infrastructure in place, Emanuel never formally declared his candidacy for a third term.

“For the last seven and a half years, I’ve given my all every day and left it all on the field,” Emanuel said Tuesday. “I’ve approached public service the only way I know how — giving 100 percent, 24 hours a day, 7 days a week.”

Last week, the Emanuel campaign chose to release results of a $153,500 poll bankrolled by Michael Sacks, the mayor’s close friend, business adviser and largest campaign contributor.

Despite an avalanche of tax increases, persistent violence and deep distrust among black voters tied to his handling of the McDonald shooting video, it showed the embattled incumbent strongly positioned to win a third term — contradicting recent polls done for competitors that showed otherwise.

The timing of the poll’s release was curious.

“They’re either trying to pump him up to run or make the case that he could have won, even if he doesn’t run,” one political operative had said at the time.

Contributing: Rachel Hinton, Michael Sneed, Fran Spielman, Lauren FitzPatrick, Tina Sfondeles, Jane Recker