Updated 10:18 p.m. ET

In an unexpected and abrupt announcement, Bill Daley, the former White House chief of staff and longtime Democratic political operative, is ending his bid for the Democratic nomination for Illinois governor.

William Daley. (Photo by Alex Wong/Getty Images for Meet the Press)

Daley served as a chief of staff for President Obama and previously served as commerce secretary, chairman for Al Gore's 2000 presidential campaign and is the son and brother of two former Chicago mayors. His decision to drop out of the race comes only four months after he announced plans to challenge Democratic Gov. Pat Quinn.

In an e-mail, Daley told The Washington Post that he "just decided this is not [the] life for me for the next five to nine years."

A campaign adviser said Daley has been wrestling with the decision for some time and was really agonizing about it. Ultimately, Daley decided "that the worst thing that could happen is that he could win," the adviser said.

The news was first reported Monday evening by the Chicago Tribune, which published an interview with Daley, who said that “One of the things I always thought in my career that I wanted to do, I thought I would be able to have that opportunity, I hoped, would be to run for office. And even though you’re around it for a long time, you really don’t get a sense of the enormity of it until you get into it."

“But the last six weeks or so have been really tough on me, struggling with this. Is this really me? Is this really what I want to spend my next five to nine years doing? And is this the best thing for me to do at this stage of my life?” Daley told the Tribune. “I’ve come to the conclusion that this isn’t the best thing for me.”

Despite dropping his bid, Daley told the Tribune that he still believes Quinn will lose to a Republican challenger: “There’s no doubt in my mind that Pat Quinn will not be the next governor of Illinois. This governor is not that strong that somebody should fear running against him.”

The Quinn campaign responded Monday night by saying it respected Daley's decision.

"A divisive primary would have only helped Republicans who want to take this state backwards and undo the important progress we have made," the campaign said in a statement. Responding to Daley's concerns that Quinn will not win reelection next year, Quinn's camp said that "when the time comes for voters to make their decision on Nov. 4 next year, we are confident they will recognize the difficult and important work the Governor has accomplished on their behalf."

In an interview with The Washington Post in August, Daley described Quinn as “a nice guy” but a weak leader who had failed to solve the state's budget problems.

“A state like ours, any state, needs a strong governor. And if there’s anything that most people, I think, would say, it’s that Pat is not a strong governor,” Daley said in The Post interview.

Here's video of the Tribune's interview with Daley, courtesy of ChicagoTribune.com: