An imminent split seems unlikely. The US remains Afghanistan's chief international backer and the Obama administration's ambitious plan to transform the war-torn country needs Mr. Karzai if it's going to succeed.
But the angry words tossed between Kabul and Washington lately have amply demonstrated the strain between a US administration that says it is committed to political reform in Afghanistan and an Afghan leader empowered by an election widely thought to have been marred by fraud.
In one recent low point, President Hamid Karzai allegedly threatened to join the Taliban if the international community kept pressuring his administration. The US State Department shot back, saying it was considering disinviting Mr. Karzai from a meeting in Washington next month. Karzai spokesman Waheed Omer said last week that Karzai never made the comments about joining the Taliban, though the Associated Press and others stand by their reporting.
"Karzai feels he's hostage to the internationals, and they feel they are hostage to him. So you have this frustration on both sides," says Martine van Bijlert, codirector of the Afghanistan Analysts Network in Kabul. "It's been like that for a long time, and it seems to be getting worse."
Yet Karzai and Washington are bound together in the interests of their charge – post-Taliban Afghanistan – with President Obama committing 30,000 more troops to safeguard Karzai's fragile regime.
Analysts are divided on how the United States might put the relationship on more constructive footing, with some advocating careful confrontation and others counseling avoidance.
What might the US do?
The confrontational approach involves warning Karzai that his anti-Western statements undermine public support from NATO democracies that have committed troops to Afghanistan.
Karzai, argues Jamie Metzl with the Asia Society in New York, should be reminded that the 18-month time frame set by Mr. Obama last year for the beginning of withdrawal is "ticking away" and when the clock strikes midnight political support for staying may disappear if his government hasn't improved.
The US could also curtail financial support to Karzai, Mr. Metzl says, and reexamine ties with figures such as his brother Ahmed Wali Karzai, who is widely considered to be paid by the Central Intelligence Agency, a charge he denies.
Metzl shares the general consensus among analysts, however, that there's no alternative to Karzai on the horizon. He says there is no serious talk in Washington of withdrawing support from Karzai as the US did with South Vietnamese President Ngo Dinh Diem in 1963, which led to Diem's ouster and murder that year.
"What the Diem experience has taught the US is [that] as flawed as your partner is now, the next guy will probably be worse," says Metzl.
Yet Karzai has been fanning fears that the US has a power play up its sleeve, portraying the US's recent courtship of Pakistan as preparation for granting it the keys to Afghanistan.
"If we come down really hard on [Karzai], all it does is confirm what he is saying: that we are trying to dictate policy [and] will turn the country over to Pakistan and their interests," says Marvin Weinbaum, an Afghanistan analyst at the Middle East Institute in Washington.
Mr. Weinbaum argues the US should try to deal with other Afghan leaders, including the ministers of Defense and Interior. "It means, in a sense, bypassing Karzai to the extent that you can."
US officials have been hinting at this since Karzai won his fraud-marred reelection and the State Department codified the idea by stressing "sub-national governance" in its January strategy document.
Karzai is sensitive to this. His latest outbursts came when the lower house of parliament rejected his effort to strip international oversight of Afghan elections. Karzai appeared to suspect that foreigners marshaled the parliament against him.
Afghanistan's government structure – crafted by the US after the Taliban's ouster – centralizes power around a strong executive, notes Christine Fair, an Afghanistan expert at Georgetown University. She doubts there's much room to work around Karzai.
But it's too early to judge the new effort since much of the US's "civilian surge" – designed to improve and extend Afghanistan's government – got sidetracked by Haiti relief efforts, she adds.
"It's clear that everything still needs to be signed with Karzai, so managing him remains the focus," says Ms. van Bijlert.
Karzai's words have disturbed many Afghans because they seem to jeopardize the reason people voted for him – that he had Western backing, says Omar Sharifi, the Kabul director of the Boston-based American Institute of Afghanistan Studies. "Everyone thought Karzai had the support of the US and that is the cornerstone of his power," says Mr. Sharifi.
Even longtime Afghan analysts like Haroun Mir are puzzled by Karzai's moves. There's even an unsubstantiated rumor, according to former UN official Peter Galbraith, that Karzai's inner circle says he has an opium habit. Speculation about Karzai's mental fitness has bubbled up before following earlier provocative anti-Western outbursts.
Some analysts say Karzai could lose advisers and see his government unravel if Afghan politicians become convinced the US is backing away from the president. "Even his staff are angry at President Karzai's comments," says Mr. Mir. "He is totally isolated."
Mr. Weinbaum says that a visit to Washington might give Karzai a good platform to step back from his words.
That said, it was a visit from Obama to Kabul that laid bare the latest tensions.
"I don't think there's much that can be salvaged" in the relationship, says Ms. Fair. "If Obama went being all cool and lovey-dovey … he would have been criticized.
"At some point you have to signal to your client that, hey, we are the ones paying your bills."