IF YOU WANT TO UNDERSTAND COUNTY POLITICS, a good way to start is by attending a grade-school music recital, preferably one featuring a piano or another instrument that sounds good by itself.

Student solo recitals usually feature at least one child who flails. Maybe the child has trouble with a difficult passage, or is lost or even stops altogether. The audience of moms and dads sympathizes with the struggling would-be musician, and yet you can also find a disapproving look on a few parents’ faces: What’s wrong with this kid? Why isn’t he more prepared? Why is she wasting my time?

Watch the county officials who appear in public before the Los Angeles County Board of Supervisors, and the same standards apply: Do they crumble under pressure? Do they walk away from the dais humiliated? Do you feel sorry for them, even as they irritate you for making you feel pity?

Chief Administrative Officer David Janssen, the man who spent the past 10 years reporting to the county supervisors, never failed the piano-recital test. He remained the coolest of customers, a virtuoso who showed intelligence and humor as his bosses sought answers about the most unpleasant problems: hospitals on the brink of closure, inmate deaths, a crowded county morgue overstuffed with corpses.

In other words, Janssen achieved a rare feat in a county where the elected supervisors, each of whom represents around 2 million people, are known for publicly shredding bureaucrats, most of them with the word “former” attached to their titles — the former health director, the former head of the child-welfare agency, and, especially, the woman who preceded Janssen. The consummate bureaucrat, Janssen was the ultimate behind-the-scenes adviser in a county government that goes ignored, largely because the bulk of its services target the poor.

“David is probably one of the least known, most powerful and influential government officials in Los Angeles,” said Jim Lott, executive vice president of the Hospital Association of Southern California, who watched an estimated 300 supervisors’ meetings and never saw Janssen taken down.

Now the 61-year-old Janssen is leaving, a move that has left a large portion of the county government distraught. One seasoned political aide compared Janssen’s pending retirement, scheduled for mid-January, to Joe DiMaggio’s departure from baseball. Others labeled him a policy genius, someone who exuded credibility as he advised the supervisors — five kings and queens who hire and fire the county’s 37 department heads.

“He didn’t tell us something on a Monday and then change it Tuesday at the board meeting,” said Supervisor Don Knabe. “You could take his word to the bank.”

One way to respond to the news of Janssen’s pending resignation is to panic. Does he know something we don’t? After all, Janssen is retiring at an upbeat time for the county. Flush with property-tax revenue from an overheated real estate market, the supervisors voted last month to pour money into the Sheriff’s Department, a new homelessness initiative, even repairs to the Natural History Museum. The county’s budget is now closely tethered to the fortunes of the real estate market. Is Janssen leaving because he knows the market is about to tank? After all, Janssen even sold his house in Pasadena to move into a rental.

“It has nothing to do with that,” Janssen replied. “We’re fine for the next several years. It won’t tank. Nobody’s predicting it will tank. It’ll slow down. Things are going well virtually everywhere except the health department, so it’s a good time to leave.”

Janssen came to Los Angeles County in 1996 from San Diego, where he had considerably more leverage over county government — that is, the ability to hire and fire agency heads, and therefore manage. The L.A. supervisors had just kept the county from sliding into bankruptcy and were shaken by the experience. With Janssen by their side, they renewed their resolve to say no to new spending requests, such as a demand from the Sheriff’s deputies’ for expanded retirement benefits. County supervisors came to trust Janssen’s recommendations, even when they disagreed with them.

“PEOPLE SAY HE’S THE SIXTH supervisor,” said Supervisor Zev Yaroslavsky. “He would bristle at that notion, but in the deliberative process, bouncing ideas around, we are six people. The five supervisors and David Janssen.”

Yaroslavsky argued that Janssen’s legacy goes well beyond performances in board meetings. Janssen and the supervisors made a series of unpleasant decisions together, slashing the county’s public-safety budget during the last recession and voting to shutter hospital facilities (a move mostly reversed by a federal judge). A resurgent economy helped the county restore public-safety spending, but Janssen said only the federal government can address the financial crisis facing the health department, which is overwhelmed by the needs of the uninsured.

Ever the diplomat, Janssen refused to take credit for the county’s more positive developments, from resurgent financial stability to the opening of various facilities, particularly Disney Hall and the Twin Towers jails — projects which stalled in the 1990s because of budget woes. The accomplishments caught the eye of Governor Arnold Schwarzenegger, who attempted unsuccessfully to lure Janssen to Sacramento to serve as the state’s new finance director, Yaroslavsky said.

Janssen’s departure should, but very likely won’t, reopen a discussion on the type of government that Los Angeles County deserves at the start of the 21st century. Yaroslavsky, for his part, contends that the county needs an elected executive to manage departments. “This is a county that . . . has a five-headed executive,” he said. “Which means everything from whether we add a bathroom on the second floor to rebuilding a county hospital, it gets to be vetted by five people.”

With Janssen’s announcement still fresh, the supervisors have not devised a search process to replace the consummate county bureaucrat. “I’m still trying to get him to change his mind,” Knabe said.

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