Afghanistan's electoral rules dictate that a candidate must obtain more than 50 percent of the vote to win a first-round election.
The Taliban has threatened to attack polling stations on Saturday, and warned this week of "decisive attacks."
In April, the Taliban similarly threatened violence and carried out some attacks to deter voters. Despite their efforts, voter turnout was high.
The winner of tomorrow's runoff will be Afghanistan's first democratically-elected president.
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