President Obama now faces a new complication: enabling a badly tarnished partner to regain enough legitimacy to help the United States find the way out of an eight-year-old war.

It will not be easy. As the evidence mounted in late summer that Mr. Karzai’s forces had sought to win re-election through widespread fraud to defeat his main challenger, Abdullah Abdullah, administration officials made no secret of their disgust. How do you consider sending tens of thousands of additional American troops, they asked in meetings in the White House, to prop up an Afghan government regarded as illegitimate by many of its own people?

The prospect of stepping up a war in Afghanistan is conjuring the ghosts of men like McNamara and McGeorge Bundy. A recent book about Bundy’s view of Vietnam is practically required reading in the West Wing these days. As in those early days of Vietnam (like Afghanistan today, a war-hardened country with a history of expelling foreign powers), no path seems especially clear or promising. The generals want an infusion of 40,000 more troops — a move that could lash Obama’s presidency to Afghanistan almost as tightly as George W. Bush’s was bound to Iraq. Liberals in Congress are equally adamant about cementing a plan for withdrawal, which would most likely lead to the return of the Taliban. (The 1960s left might have believed that the Viet Cong was, in fact, a people’s uprising, but no one can make that case about the Taliban, whose violent repression of women and nonbelievers while in power alienated most Afghans and shocked the world.) Judging from a recent poll by ABC News and The Washington Post, war-weary American voters are divided almost evenly on a course of action.

“We congratulate President Karzai on his victory in this historic election,” said a statement from the United States Embassy in Kabul, “and look forward to working with him, his new administration, the Afghan people and our partners in the international community to support Afghanistan’s progress towards institutional reforms, security and prosperity.”