The late Benazir Bhutto's party has named its candidate for prime minister, after winning last month's elections.
The Pakistan People's Party nominated ex-Speaker Yusuf Raza Gillani for the post to lead a coalition government with ex-PM Nawaz Sharif's PML(N) party.
Analysts say the appointment is widely expected to be a stop-gap measure and Ms Bhutto's widower, Asif Ali Zardari, may be poised to eventually take over.
The coalition is expected to try to curb President Musharraf's powers.
Mr Gillani is virtually guaranteed to win a parliamentary vote to make him PM on Monday because of the coalition's massive majority, says the BBC's Barbara Plett in Islamabad.
Ms Bhutto's son was said to have been feeling unwell by party officials
A party leader from southern Punjab, he is admired within the PPP for going to jail in 2001, rather than doing a deal with Mr Musharraf.
It is believed he will quietly step aside if Asif Ali Zardari, the PPP's effective leader and co-chairman, decides to go for the top job, says our correspondent.
Ms Bhutto's widower is not eligible to be PM as he is not currently an MP but he may stand at a by-election within months for a seat in the National Assembly, analysts say.
Saturday's announcement had been due to be made by Bilawal Bhutto Zardari, the son of the late PPP leader who was assassinated at an election rally in December. But the PPP said the teenager was not feeling well.
Our correspondent says there is speculation the plan was changed to spare the 19-year-old tough questions about reports of bitter behind-the-scenes wrangling over the nomination.
Ms Bhutto's widower is still the PPP's most powerful figure, say analysts
The vice-chairman of the PPP, Makhdoom Amin Fahim, had long been considered the favourite to lead the new government, says our correspondent, but was apparently sidelined.
Some party members say Asif Ali Zardari saw Mr Fahim as a rival power base in the PPP, others that Mr Fahim is seen as too close to President Pervez Musharraf.
Mr Zardari and his main coalition partner, Mr Sharif, have told The New York Times they are prepared to negotiate with the militants, reflecting a coalition consensus on the need for a comprehensive political approach to Islamist violence.
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Ms Bhutto's party has been in intense talks over the make-up of a coalition administration since winning the 18 February elections.
President Musharraf, a US ally who came to power as a general in a 1999 coup, suffered heavy losses in the polls and appears increasingly isolated, say analysts.
The new coalition government has vowed to reinstate judges sacked by the president during a state of emergency in November.
Correspondents say the judges, if restored, could overturn Mr Musharraf's re-election in a parliamentary vote last October, effectively ruling his presidency illegal.