So, it seems that PPP memebers are wary that Nawaz Sharif is only interested in grabbing the ‘Kursi’ from right under Zardari’s PPP.
I must admit Nawaz Sharif is being very very clever. I guess he has learnt from his humiliation at the hands of BB when she tossed him aside to start negotiatiins with great President Musharaf. 
http://www.dailytimes.com.pk/default.asp?page=2008/02/29/story_29-2-2008_pg7_56
PPP members wary of manipulation by PML-N
Say PML-N wants to lift ban on becoming prime minister thrice
LAHORE: Pakistan Muslim League-Nawaz’s (PML-N) demand for the restoration of the 1973 Constitution as on October 12, 1999, aims in guise at lifting a ban on becoming a prime minister thrice, a number of Pakistan People’s Party (PPP) legislators-elect told Daily Times.
Of the present lot of politicians in the race for the top executive office, Mian Nawaz Sharif is the only victim of the constitutional bar on a third term as prime minister. This bar cannot be lifted without a two-thirds majority in the parliament, which can only be achieved with the support of the PPP, the Awami National Party (ANP) and independent candidates.
The PML-N is pressing for the removal of President Pervez Musharraf, restoration of the 1973 Constitution and reinstatement of all those judges who did not take oath under General Musharraf’s November 3, 2007, Provincial Constitutional Order (PCO).
One PPP legislator-elect from Sargodha division told Daily Times that there was a strong feeling among his colleagues that the actual purpose of the PML-N was to get rid of the constitutional clause banning a person from becoming a prime minister thrice.
There is visible apprehension among the PPP parliamentary party that the PML-N’s attitude would change once the clause is removed from the constitution.
Another PPP legislator-elect from Gujranwala division told Daily Times that Nawaz Sharif had made it too difficult for the PPP to evade the PML-N’s demands by taking a high moral position on various issues, although some of the demands harmed the PPP’s political interests.
There is a feeling among the PPP legislators-elect that the PML-N could leave them once its objectives are achieved with their cooperation. To them, the PML-N objectives include election of Shahbaz Sharif as the chief minister of Punjab, lifting a ban on a third term as prime minister, and reinstatement of Justice Iftikhar Muhammad Chaudhry as the chief justice of Pakistan.
The PML-N leadership’s firmness on its stance to extend parliamentary support to the PPP government in centre without joining the cabinet and Chaudhry Nisar Ali Khan’s news conference in Islamabad last Tuesday is proving to be a major source of unease among the PPP legislators-elect.
Chaudhry Nisar had said in his news conference that the PML-N would see how the PPP’s government would respond to his party’s demands during first three to six months before deciding if it would accept ministries in the federal cabinet.
According to one legislator-elect, PPP Co-chairman Asif Ali Zardari is upset over PML-N’s attempts to gather support of independent candidates to enhance his party’s strength in the parliament, as it was against what the two leaders had decided during their first meeting at Nawaz Sharif’s Lahore residence after the elections.
Zardari expressed concern over the issue during a party meeting in Islamabad last week, said the legislator-elect.
According to an insider, Zardari got irritated by the Musharraf government’s attempt on February 20 through its lawyers to press the Swiss court to prosecute him in a money-laundering case. But the Supreme Court’s decision on Wednesday that the National Reconciliation Ordinance (NRO) was valid had calmed him down to a large extent, he said. “This can lead to some dramatic developments for future power-sharing scenario,” the insider said.