Winning control of the Senate could enable the Pakistan Muslim League-Nawaz (PML-N) to change the constitution to make former prime minister and party leader Sharif eligible to hold office again when the party contests a national election due later this year.
While PML-N has not spoken to amending the constitution in such a way, the party has been on a collision course with the country's judiciary since July when the Supreme Court ousted Sharif from power by disqualifying him over undeclared assets.
Last month the top court again disqualified Sharif by removing him as head of PML-N, the political party he founded.
A total of 135 candidates, which includes 25 women, are contesting the 52 senate seats. The elected senators would serve a six year term.
The opposition Pakistan Peoples Party (PPP) currently controls the 104-seat senate but is expected to be replaced by the PML-N as the single biggest party. With its allies in the senate, PML-N is tipped to take control of the chamber.
Sardar Ayaz Sadiq, the speaker of the National Assembly, cast the first vote in the elections in capital Islamabad, the parliamentary media wing said on Saturday morning.
Polling is taking place in Pakistan's four provincial assemblies and the federal parliament, with officials expecting final results to be announced later in the afternoon.
The final results will not immediately show how well PML-N has done because a number of candidates were barred from running under the party's banner following Sharif's disqualification by the Supreme Court last month.
Those candidates, who will be running as "independents", are expected to pledge their allegiance to PML-N if they are elected to the senate, PML-N party officials say.Under Pakistan's constitution a bill cannot become law unless it is passed by both upper and lower houses of parliament.
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