The departure of Rahm Emanuel, the White House chief of staff, to run for mayor of Chicago marks, more than any other single event, the end of the first chapter of the Obama presidency.
The hard-driving Mr Emanuel was chosen for arguably the most pivotal job in the American system of government. His aggressive style, combined with his previous experience at the Clinton White House and later as part of the Democratic leadership on Capitol Hill, made him the ideal person to force through Mr Obama's ambitious legislative agenda.
There is a pattern to American presidencies. A president's power to have his way with Congress is never greater than early on, when memories of his national electoral mandate are fresh, before scandals and the inevitable missteps sap his popularity and his authority. So it has been with this White House.
Mr Emanuel's tenure saw the passage of the largest economic stimulus package in US history, the most sweeping overhaul of healthcare in half a century, and new financial regulation for Wall Street – all in the teeth of unyielding Republican opposition in both the House of Representatives and the Senate. Now the initial activist phase is over, and Mr Obama's energies for the next two years will be focused less on changing the country than securing his own re-election in 2012.
Mr Emanuel's going is not a moment of crisis for the President. Burnout comes quickly in the White House pressure cooker, and chiefs of staff rarely serve a full presidential term. Probably he would have stayed longer, but the job of mayor – one to which Mr Emanuel had long and openly aspired – suddenly came open, and he threw his hat into the ring. But the changeover also presents Mr Obama with an opportunity. The quiet-spoken Pete Rouse, Mr Emanuel's temporary successor, will bring a different, less frenetic, tone to the White House. However, Mr Rouse too is part of the President's tight-knit inner circle. It makes sense not to bring in an outsider now, with mid-term elections just a month away. But with other top aides also departing, Mr Obama should act quickly afterwards to bring fresh blood to the White House.