In August, U.S. Secretary of State John Kerry, left, spoke as Afghan presidential candidates Ashraf Ghani Ahmadzai, center, and Abdullah Abdullah listened during a joint press conference in Kabul, Afghanistan. (Rahmat Gul/AP)

The prospect of two candidates declaring themselves the elected successor to Afghan President Hamid Karzai grew significantly Monday, threatening the Obama’s administration’s efforts to prevent the country from erupting in political unrest.

After weeks of recriminations over the disputed results of a June runoff election and negotiations on forming a unity government, Abdullah Abdullah declared victory on national television and pledged to block his rival from taking power through “fraudulent results.” The defiant statement heightened tensions days before the announcement of audited election results, which are expected to deliver the presidency to former finance minister Ashraf Ghani.

‘“We are the winner of the election based on the clean votes of the people,” said Abdullah, claiming that the vote was plagued by widespread fraud. “Fraud, fraudulent results and the announcement of the fraudulent results are not acceptable.”

In response, Daoud Sultanzoy, a top aide to Ghani, said Ghani is prepared to assume power unilaterally should Abdullah fail to return to the bargaining table.

“This is not about a spoiled group that wants to keep a grip on power,” said Sultanzoy, noting the stalemate is hurting the economy. “This is about the people of this country, and we are cognizant about this and won’t be reckless.”

The standoff is again testing the patience of President Obama as he seeks to nudge Abdullah and Ghani into a unity government to succeed Karzai. On Saturday night, Obama called both candidates and urged them to follow through on a power-sharing agreement that Secretary of State John F. Kerry brokered this summer. Kerry had also spoken with both candidates several times during the past few days, including Monday.

But talks between Abdullah and Ghani on the formation of a unity government — one candidate would serve as president and the other in a newly created chief executive position — broke down Monday morning. A few hours later, Abdullah announced the “deadlock” and said he could not accept a Ghani government.

In Washington, State Department spokeswoman Jen Psaki said the administration remains hopeful a crisis can be averted.

“Certainly our preference would have been to resolve this quite some time ago, but obviously we have to work in the situation that we’re in, and so our effort is to determine how we conclude this through the electoral process,” Psaki said.

If Ghani and Abdullah both claim victory, billions of dollars in international assistance for Afghanistan could be at risk as donors grow more skittish about the long-term stability of the country. It could also set the stage for Karzai to extend his term or lead to calls for the formation of an interim government.

At stake are U.S. plans to keep up to 10,000 American troops in Afghanistan in 2015 after most other NATO forces pull out by the end of this year.

Karzai, who was installed as president in 2001 and went on to win successive elections, has refused to sign a long-term security agreement with the United States. Ghani and Abdullah have pledged to sign it if elected, but it appears that weeks could pass before either is sworn in as Karzai’s replacement.

“I think they are running out of time,” said Andrew Wilder, vice president of the Center for South and Central Asia at the United States Institute of Peace. “Afghanistan is going to need to work hard to maintain the diminishing support it has from the international community, and it’s not going to be able to do that without a president.”

Many observers and Western officials say the country missed a huge opportunity last week, when the prolonged recount meant neither Abdullah nor Ghani represented Afghanistan at the NATO summit in the United Kingdom; the defense minister was there instead. Wilder noted another diplomatic deadline is approaching later this month, when the world leaders gather for the United Nations General Assembly in New York.

An equally immediate concern is whether the stalemate could lead to violence between supporters of the two men, perhaps even splintering the country along ethnic lines.

Abdullah was a top aide to legendary Afghan guerrilla commander Ahmed Shah Massoud, who was assassinated by al-Qaeda operatives two days before the Sept. 11, 2001, terror attacks.

Abdullah’s mother is of Tajik descent and his father was Pashtun, but Abdullah appeared to receive most of his support from predominantly Tajik areas of northern Afghanistan, where Massoud, his late mentor and an ethnic Tajik, was especially revered.

Ghani, who is Pashtun, received his greatest support among Pashtuns, Afghanistan’s largest ethnic group.

In his address Monday, Abdullah repeatedly urged his followers not to resort to violence. Abdullah said he continues to be “patient,” suggesting that he still could be persuaded to rejoin talks with Ghani.

But Kabul is on edge because Tuesday is a public holiday when Afghanistan marks the day that Massoud was assassinated. The day typically involves large rallies, but there are fears here that they could turn into protests in support of Abdullah.

In recent days, Western diplomats have stepped up pressure on supporters of both men in an attempt to limit tensions.

Attah Mohammed Noor, a powerful governor and former militia leader from northern Afghanistan, told The Washington Post last month that there would be a “big civil uprising” should Abdullah be denied the presidency because of perceived fraud. During the weekend, a visiting delegation from Germany, which has extensive investments in northern Afghanistan, met with Noor and told him such talk is harmful to Afghanistan, according to a senior official with the international coalition.

Still, Abdullah repeatedly stressed Monday that he and his supporters cannot be expected to accept an election they view as fraudulent.

Abdullah, a former foreign secretary, finished first in a field of eight candidates in the initial round of voting in April but fell short of a majority. In the runoff against Ghani, however, Abdullah lost by more than 1 million votes.

Sharif Hassan and Sayed Salahuddin in Kabul and Anne Gearan in Washington contributed to this report.