Former Prime Minister Nawaz Sharif has pulled his party out of Pakistan's governing coalition.
At a news conference in the capital, Islamabad, Sharif blamed the main ruling party of Asif Ali Zardari, the Pakistan People's Party, for the breakup of the five-month-old alliance.
Sharif said Zardari's party had failed to restore judges ousted by former President Pervez Musharraf according to a timetable they had agreed on. Sharif said the two coalition partners also had failed to agree on a neutral successor to Musharraf, who resigned a week ago.
"We therefore feel that these repeated defaults and violations have forced us to withdraw our support from the ruling coalition and sit on the opposition benches," Sharif said.
"However, we will play a constructive role in light of [the] charter of democracy and continuous struggle for restoration of judges and genuine democracy in Pakistan."
Zardari has been nominated by his party as a candidate for the president. Sharif has named a retired judge to run against Zardari when Pakistan's electoral college names the next president on September 6.
The collapse of the coalition is expected to concentrate power in the hands of political parties that are seeking to preserve Pakistan's alliance with the United States in its struggle against Islamist militants based in the tribal regions along the Afghan border.
"It basically means a realignment within the Pakistani political spectrum," says Ayaz Khan, a broadcaster with RFE/RL's Radio Free Afghanistan who specializes on Pakistan.
"Pakistani politics have been dominated by the military for the past nine years, since ex-President Musharraf took over in a bloodless coup in 1999. After his resignation, the sense is that the military is not going to stage another coup anytime soon, given that the current head of the Pakistani military has indicated that the military will keep out of politics."
Khan concludes that issues far deeper than the reinstatement of judges or different choices for the next president are at play.
"There are many reasons for Sharif's pullout. The obvious reason is, of course, the judges issue. Sharif and Zardari agreed to restore the judges that Pervez Musharraf fired last year on November 3 when he declared an emergency in the country," he says.
"But the other reasons are that there are larger ideological differences and political differences. One other reason which many people are still speculating about is the war on terror. The two do not agree on how to pursue the war on terror."