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Longtime Chicago Mayor Richard Daley announced Tuesday that he will not seek re-election, a stunning decision that apparently ends a political dynasty that dominated the city for more than a half century and paves the way for Rahm Emanuel to chase his dream job, if he chooses.

The nation's third-largest city has been ruled by a "Mayor Daley" for 42 of the last 55 years, beginning with Richard J. Daley, who ran Chicago and its Democratic machine for two decades until his death while in office.

Cook County Clerk David Orr said he did not think anyone would have seriously challenged Richard M. Daley, the elder Daley's son, if he had run for re-election. But Tuesday's announcement means "the whole political landscape changes enormously," Orr said.

Emanuel, the White House chief of staff, once told Fox News that if Mayor Daley, 68, didn't run in the February election, he might throw his hat in the ring, raising the possibility that his days in the White House are numbered.

"That's a job I really want," Emanuel told Fox News' Bret Baier.

But Emanuel didn't offer any hints that he was still interested Tuesday after Daley's announcement.

"While Mayor Daley surprised me today with his decision to not run for re-election, I have never been surprised by his leadership, dedication and tireless work on behalf of the city and the people of Chicago," he said in a written statement.

White House senior adviser David Axelrod said Tuesday that Emanuel wasn't the only one in the White House caught by surprise by Daley's announcement.

"We both strongly assumed he was (going to run). So we're just absorbing the news," Axelrod said. But he wouldn't comment on whether Emanuel will run for mayor.

"Rahm...has a lot on his plate. He's got a pretty responsible job, and that's what he's focused on right now," he said.

Daley was first elected mayor in 1989, following in the footsteps of his father, who died of a heart attack while still mayor in 1976 at age 74.

During his tenure, the younger Daley presided over some of the most dramatic changes in Chicago history.

He assumed command of the floundering school system in 1995 and replaced entrenched bureaucrats with tough, results-oriented administrators. Homework became mandatory, and the "social promotion" of underperforming students was halted. Test scores climbed steadily.

Daley also was the catalyst for a citywide facelift. West Side slums were cleared, new green space was created, a theater district came to life in the north Loop, and Navy Pier became a colorful playground complete with boat rides and a giant Ferris wheel.

Critics have grumbled that in some ways Daley's Chicago was run much as it had been under his father, who was the boss of Chicago's Democratic machine for two decades. They pointed to City Hall scandals and lucrative contracts for the mayor's friends as well as chronic corruption and police brutality cases.

Daley's constituents came to know him as an intense, nervous figure who was never fully comfortable on the speaker's platform. As with his father, the current mayor's rhetoric is at times tangled.

He nevertheless remained popular, winning elections by overwhelming margins.

The fourth of seven children and the oldest son of Richard J. and Eleanor "Sis" Daley, Richard M. Daley grew up in the 11th Ward near the former Comiskey Park, an area of blue-collar bungalows and two-flats, home to many city patronage jobholders as well as judges, prosecutors and police officers.

Politics was a part of family life. A brother, William Daley, would become U.S. commerce secretary under President Clinton and another, John Daley, is a Cook County commissioner.

In 1972, Richard M. Daley was elected to the state Senate. In 1980, he was elected Cook County state's attorney.

In 1983, Daley finished third in a mayoral primary marred by racial antagonisms. U.S. Rep. Harold Washington defeated Daley and Mayor Jane M. Byrne and became the city's first black mayor. In 1987, Washington died of a heart attack, and Daley won a 1989 special mayoral election.

At a news conference Tuesday, Daley said he had been thinking about his decision for several months and that "it just feels right."

"In the end this is a personal decision, no more, no less," Daley said.

The mayor's wife Maggie has been battling breast cancer. She has made a few public appearances but does not talk publicly about her health.

Daley did not take questions after making the announcement Tuesday.

President Obama issued a brief statement on Daley's announcement.

"No mayor in America has loved a city more or served a community with greater passion than Rich Daley," Obama said in a written statement. "He helped build Chicago's image as a world class city, and leaves a legacy of progress that will be appreciated for generations to come."

The decision immediately raises speculation about whether the 50-year-old Emanuel will make a move for the seat.

Emanuel, a one-time Daley adviser and a Chicago native, said in an interview in April that if Daley did not seek another term, he would like to run for the office.

"If Mayor Daley doesn't (run), one day I would like to run for mayor of the city of Chicago. That's always been an aspiration of mine even when I was in the House of Representatives," Emanuel told PBS' Charlie Rose. Emanuel was an Illinois congressman until he resigned to take his current White House post.

Others who might launch a bid for the job include Illinois Reps. Luis Gutierrez and Jesse Jackson Jr., who said in a statement that he expects "numerous candidates, but only a few can mount a serious bid for mayor."

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The Associated Press contributed to this report.