President George W. Bush began the year in control of the commanding heights of American politics. Re-elected for a second term, uninhibited by having to face voters again, his party in control of Congress, he looked forward to four years in which to pursue his personal agenda and leave his stamp on the nation. He is now looking more like a political cripple, under siege from Republicans and Democrats alike, after a week of events that threaten to leave the world's only superpower with a lame-duck leader for the next three years. Mr Bush is the latest victim of the second-term plague that has struck every two-term president since Dwight Eisenhower 45 years ago. Like Mr Bush, all were hit by criminal investigations during their second terms. Richard Nixon resigned over the Watergate scandal, Ronald Reagan was weakened by the Iran-Contra scandal and Bill Clinton was impeached for perjury in the Monica Lewinsky affair. The White House is reeling from the departure of top aide Lewis 'Scooter'' Libby after he was indicted on charges of obstructing justice and lying in the investigation of the leaking of a covert CIA agent's identity. In the same week the United States death toll in Iraq topped 2,000 and a revolt in Mr Bush's conservative Christian political support base forced the withdrawal of his controversial nominee for the Supreme Court bench - Bush family lawyer Harriet Miers. Things could have been worse. The indictment did not touch the president's chief political adviser, Karl Rove, who remains under a cloud in the same investigation. It is nonetheless a reminder of the administration's discredited justification for the Iraq invasion. The 'outing' of CIA agent Valerie Plame apparently was retaliation for her husband's dissent from the administration's case for war. The unfounded claim that Saddam Hussein had weapons of mass destruction continues to undermine the president's credibility. His approval ratings have hit an all-time low. They have been battered by rising anti-war sentiment, the administration's inadequate response to the Hurricane Katrina catastrophe in New Orleans and a perception, driven by soaring health-care and fuel costs, that the economy is not doing as well as the experts say. The taint of cronyism, dishonesty and incompetence is palpable. The Bush White House faces a defining challenge. The president's projected image of good, strong leadership is in tatters. The nomination of a replacement for Ms Miers for the Supreme Court presents the opportunity to begin work on repairs. That rules out the nomination of yet another crony. Politically it will be difficult to resist choosing someone with a record as a conservative on social issues. Anything else would risk further alienating the Christian right ahead of mid-term congressional elections next year. The nominee should be beyond reproach judicially. Even then, he or she faces a bruising confirmation process from Democrats who are baying for Mr Bush's blood. He must also seize the opportunity presented by the recent successful referendum on an Iraqi constitution to press for further reforms to make the final result acceptable to all three of Iraq's main religious and ethnic groups - the only workable basis for national unity and development and eventual American withdrawal. It will take more than luck to recover from the worst week of his presidency. But Mr Bush has shown the ability to change course. For instance, a recent reaffirmation of strong support for the United Nations contrasts with the unilateralism that led to the invasion of Iraq without security council backing. And a recognition that poverty is the root of terrorism has been followed up with a proposal to slash agricultural tariffs to help poor nations - which has put pressure on Europe to follow suit. These are isolated examples. But they are encouraging, because weak leadership from the White House does nothing for a troubled world.