ISLAMABAD -- Members of Pakistan's provincial and national assemblies started voting on Tuesday to elect national senators amid a new set of controversies in the nation's elections.

The Senate, the upper house of Parliament, has to pass all legislation before it can become law. Each of Pakistan's four provinces elects 23 senators, and the lower house chooses another four representing the Islamabad Capital Territory, for a total of 96. Four more current seats for former tribal areas are set to expire. Senators serve six-year terms, and elections are held every three years for half of the total membership.