The No. 2 official at the Homeland Security Department, Deputy Secretary Michael Jackson, is leaving his post for financial reasons, he announced in an e-mail to colleagues today. Jackson has been at DHS since March 2005. He said he’ll leave next month.

I interviewed Jackson in May about his efforts to prepare DHS for the upcoming presidential transition. The department has been plagued by turnover at the senior most levels, a fact that Jackson acknowledged, and experts worry that this makes the department especially vulnerable in the normally rocky hand-off from one administration to another.

“We’ve had a significant turnover,” Jackson said. “And that turnover has been below the top-level jobs as well.”

Some of Jackson’s other quotes about DHS’ personnel issues seem rather ironic now, in light of his decision to leave.

• “We’re trying to nurture a cadre of owners. I am the part-time help at DHS.”
• “I won’t blow smoke at you and say everything is nailed down and perfectly fixed. The day that someone in my department tells you that about DHS is the day that person should get out of his job.”
• “If a day goes by and I don’t use up some of my brain cells focusing on this [transition] problem, it’s a very unusual day.”

Jackson insisted that the transition was “not something I feel anxiety about.” But employees and DHS watchers are likely to feel a mix of anxiety and maybe some relief with this changing of the guard. On the one-hand, Jackson was managing the sprawling department day-to-day, along with an army of lieutenants. His exit leaves an important vacancy at the very top, which officials will, presumably, scramble to fill.

But others might welcome the change. Some of Jackson’s critics have accused him of micro-managing decisions, and not yielding enough authority to his subordinates. Those critics say that DHS could operate more efficiently with a lighter touch.

Regardless of how Jackson’s departure is greeted, though, one thing is sure: This puts DHS in a precarious position. The department desperately needs strong leadership, and its hierarchical structure necessitates that it come from the top. The new deputy may change that, but for now, it’s the way things are.

As an aside, political strategists will likely see no coincidence between the timing of Jackson’s departure and the nomination of Michael Mukasey to be attorney general. When Alberto Gonzales announced his intention to resign, the early betting was on DHS Secretary Michael Chertoff to replace him. Presumably, had Chertoff become AG, Jackson would have moved up to secretary at DHS. I don’t know what financial concerns were behind Jackson’s decision to depart, but many will presume that when it was clear he wasn’t being promoted, that fact figured into his calculus.

For his part, Chertoff had this to say about his departing colleague in a press release.

Michael will leave this department having made an enduring impact on our homeland security. At this department, he was fundamental in invigorating our operating components, fusing our intelligence capabilities, building a new FEMA, and managing the response to the disrupted airline plot of August 2006. He brought tremendous focus, discipline and planning to department-wide operations, budgets and polices, and he significantly advanced the integration of our component agencies. Michael kept an open door for all 208,000 employees, and was relentless in building with them a common department culture. His work has earned him wide respect throughout the Congress, with state and local officials and among international allies.

Our homeland is more secure and better prepared as result of Michael’s tireless service, and on behalf of all Americans, I offer him our deep gratitude. I respect and admire his difficult decision to move on, and I look forward to our continued friendship.