U.S. frustration at Karzai's failure to rein in corruption, which feeds Taliban gains, has been building. It peaked over the blatant rigging of the presidential election in August.

Vice President Biden and Richard Holbrooke, special envoy to Afghanistan and Pakistan, have confronted Karzai in public, with no results. So it's been fascinating to watch the impact of the softer approach tried on Obama's behalf by Sen. John Kerry (backed up by Secretary of State Hillary Clinton's phone calls).

Kerry, who was on a previously scheduled trip to Kabul, spent several days closeted in private with Karzai last week. Whatever he said persuaded a reluctant Karzai to agree to a Nov. 7 runoff election against his closest competitor. So is it possible that a reset of U.S. relations with Karzai is under way?

"I am convinced President Karzai understands the need to make some changes," Kerry told the Council on Foreign Relations on Monday.

One can imagine Kerry's words to the Afghan leader. No doubt the Massachusetts Democrat stressed that Congress and the U.S. public won't support an extended American troop presence unless Karzai changes course.

Kerry even said he talked with Karzai "about the perceptions of his brother," Ahmed Wali Karzai. The younger Karzai is a senior official in the key, Taliban-ridden province of Kandahar, and is often accused of heavy involvement in the opium trade. Kerry said he'd sought information from U.S. intelligence sources on Ahmed Wali's alleged drug links, but "nobody has [shown him] the smoking gun."

Kerry went on to say - in diplomatic mode - that Armed Wali Karzai had done things "that haven't been helpful" and other things "that are very helpful to us. And we need to look hard at the balance of how we can best manage Kandahar.

"I am confident that is a conversation that is going to be engaged in very, very soon with the president," Kerry continued. "And it ought to be done at that level. ..." In other words, everything will be on the table - but in private, high-level talks.

Here's what's so crucial about Kerry's intervention: If the Nov. 7 elections are indeed held, Karzai will almost certainly win them. So it is essential to repair the U.S. relationship with him, which has gone very sour.

Clearly, open threats don't work; Holbrooke's and Biden's public anger appears to have solidified Karzai's belief that the United States wants his ouster. Yet the clear message Kerry delivered privately seems to have made an impact.

"I'm sure President Karzai has heard this message [about corruption] loudly and clearly from the United States and the Europeans and, more importantly, from Afghans," said Said T. Jawad, the Afghan ambassador to Washington.

Jawad told me that if Karzai wins, he will lay out two compacts: one with the Afghan people, in which he will pledge to clean house, and one with the international community, in which he will detail his plans to improve governance.

As for U.S.-Afghan ties, the ambassador added, "If there is a relationship of trust on both sides, U.S. officials can convince the president. That's the kind of relationship we need, for your ambassador to sit down quietly." I presume this means no more shouting from Ambassador Holbrooke, please.

Skeptics may wonder whether Karzai can change or become less beholden to regional warlords if he is reelected. However, the Afghan leader is more likely to embrace those warlords if he fears U.S. officials are trying to sideline him or are planning a speedy exit from his country. No matter how frustrating the relationship with Karzai, it must be repaired.

That doesn't mean U.S. officials should refrain from candor. The message Kerry carried must be repeated over and over: Obama won't be able to maintain political support for a robust Afghan strategy unless Karzai shifts course. And if we leave, Karzai will be in big trouble.

However, to be believable, that message will need to be delivered from the top, not just by emissaries. "We need to get Karzai off the idea that [the United States] is going to oust him," said former U.S. ambassador to Afghanistan Ronald Neumann. "President Obama should invite Karzai to Camp David for a couple of days. Karzai likes to walk, so walk with him. He needs to feel we're not out to get him."

A walk in the woods in which Obama listens closely, then lays out what Karzai must deliver, may be just what the U.S.-Afghan relationship needs.