Updated

Former White House chief of staff William Daley abruptly dropped out of the race for Illinois governor Monday, signaling he wasn't sure he wanted to devote time to a long, hard campaign followed by at least four years in office.

The decision leaves Gov. Pat Quinn without a prominent challenger in the Democratic primary next March, unless another candidate makes a late entry into the race.

Daley spokesman Peter Giangreco confirmed that the son of late Mayor Richard J. Daley and brother of former Mayor Richard M. Daley was ending what many had believed would be a serious challenge to the incumbent. Giangreco offered few details, saying more information would be offered Tuesday morning at a news conference with the Chicago Democrat.

Giangreco said only that Daley, 65, had been rethinking the decision about committing to a hard campaign and then one or two terms in office, as well as "what it's going to take" to dig Illinois out of its massive financial problems.

It would have been the first elected office for Daley, though he had served as a trusted adviser to two presidents, first as Bill Clinton's commerce secretary and then as Barack Obama's chief of staff after Rahm Emanuel left the post to make a successful run to succeed Richard M. Daley as mayor.

"There's nothing that prepares you for getting into these things," Giangreco said.

The decision was first reported by the Chicago Tribune. In a videotaped interview on the Tribune's website, Daley says that even if one is around politics for a long time, "you really don't get a sense of the enormity of it until you get into it."

The decision comes less than four months after Daley said he would challenge Quinn and aggressively criticized the governor for his handling of the state's nearly $100 billion public pension shortfall and other issues. Daley was the only Democrat to announce a challenge to Quinn, after Illinois Attorney General Lisa Madigan announced in July that she would not to run for the state's highest office and seek re-election instead.

With name recognition, a burst of hefty fundraising, big name supporters and several statewide tours, Daley had been gearing up to mount a serious campaign. His platform had begun to emerge with promises to reform campaign finance in the state. He also had promised to come up with more details on ways to solve Illinois' pension crisis and backlog of unpaid bills.

Political analyst Thom Serafin called Daley's decision to drop out of the race an "unbelievable turn of events for Quinn."

Serafin says the move gives Quinn the opportunity to unify Democrats and raise funds for the race against one of four Republicans in the race.

"Daley represented a formidable obstacle for Quinn," he said. "He's got an open road now. He can take his time in making decisions for his campaign, and he has more time to focus on governing the state."

Quinn's campaign put out a statement 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.

Recently Quinn had stepped up his campaign rhetoric against Daley, taking shots at his banking background and expertly tailored suits. Last week, the governor called Daley a "millionaire banker" who "helped drive the American economy into a ditch and created the Great Recession."

"We don't particularly need advice from people who created the mess in the first place," he added.

Daley's move comes just days before Illinois Democratic leaders were expected to meet in Springfield to discuss slating party candidates for statewide office. The Cook County Democratic Party, which has historic ties to the Daley family, recently endorsed Quinn for 2014, but Daley dismissed the move, saying it was based on the power of incumbency.

The four Republicans in the race are state Treasurer Dan Rutherford, state Sens. Bill Brady and Kirk Dillard and venture capitalist Bruce Rauner.

Daley would have been just the latest son of Richard J. Daley to run for office. His brother Richard served as mayor for 22 years, longer than any other mayor in city history, including his father, before retiring in 2011. Brother John Daley is a longtime member of the county's board of commissioners.

William Daley was a Midwest chairman of JPMorgan Chase & Co. and sat on boards of several big companies including Boeing Co.

Also a successful attorney, Daley spent years as a Democratic operative, working both publicly and behind the scenes in city, state and national politics. Co-chairman of the host committee for the 1996 Democratic National Convention, Daley was tapped by Clinton in his first term to push the North Atlantic Free Trade Agreement through Congress. After the election, he was named secretary of commerce, serving from January 30, 1997, to July 19, 2000.

In the Obama administration, when Richard M. Daley decided not to seek a seventh term as mayor of Chicago and Emanuel decided to step down as White House chief of staff and run to succeed him, Obama selected William Daley to succeed Emanuel.

Daley announced his resignation in January 2012, just a year after taking the job.