ISLAMABAD, Pakistan (CNN) -- Pakistan's ruling coalition split on Monday after former Prime Minister Nawaz Sharif withdrew his party and vowed to nominate a rival presidential candidate.
The political upheaval comes one week after the coalition, which took power in March, successfully forced President Pervez Musharraf to resign.
The pullout brings further uncertainty to Pakistan, which possesses nuclear weapons and is a key ally in the "war on terror," fighting Taliban militancy and al Qaeda.
Sharif, who heads the Pakistan Muslim League-N (PML-N), had set Monday as a deadline for the government to honor its promise to reinstate dozens of judges fired by Musharraf last year. Watch how coalition has collapsed »
The Pakistan People's Party (PPP), which leads the coalition, believes the coalition should focus on picking a successor for Musharraf before it decides on reinstating the judges.
Announcing his party's withdrawal at a news conference on Monday, Sharif also voiced his disapproval with the nomination of PPP co-chair Asif Ali Zardari as the coalition's presidential candidate.
"It was agreed that there would be a nonpartisan candidate for President's office, which has been violated," Sharif said. "We therefore feel that these repeated violations have forced us to withdraw our support for the ruling coalition and sit on the opposition benches."
Sharif said his PML-N would nominate former Chief Justice Saeed Uz Zaman Siddiqui as its presidential candidate for the September 6 election.
PML-N was the junior partner in the ruling coalition, led by the PPP. Both parties were rivals in the 1990s, but joined forces after winning an overwhelming victory over Musharraf's ruling party in February's parliamentary vote.
Candidates have until Tuesday to indicate whether they will run.
The PPP met Friday and decided to nominate Zardari, the widower of former Pakistani Prime Minister Benazir Bhutto, who was assassinated during a campaign rally in December. Zardari accepted the nomination the next day.
Sharif, in a recent interview with the Wall Street Journal, said "it will be a bad day for democracy" if the judges are not restored.
"If the judges are not restored, we will perhaps be forced to sit in the opposition," Sharif said in the interview, published Thursday.
"We will not try to bring the government down. But of course then we have no choice but to sit in the opposition."