WASHINGTON — Each night, Hillary Clinton’s data experts head to a conference room on the 11th floor of her Brooklyn headquarters to start counting votes.

The sessions in the “early voter boiler room” stretch into the early hours of the morning. The team pores over turnout patterns in states where advance voting is already underway, projects how many votes Clinton and Republican Donald Trump have already received, and updates targeting lists of voters they still need.

For Clinton, October is the month when she’s likely to win or lose the election, not the official Election Day on Nov. 8. By the third week of this month, her campaign hopes to have a solid enough sample of the early vote to know whether the Democrat is on track to win the White House.