The race between Democratic congressman Ron Barber and Republican challenger Martha McSally is expected to head to a recount on Monday after Arizona’s secretary of state certifies the ballots. The race remains the only undecided congressional race not in a run-off.

Heading in to the day, McSally, a retired Air Force colonel, holds a 161-vote lead, falling within the margin to trigger a recount under state law. Last week, the Barber campaign attempted to cut in to her lead by filing a lawsuit challenging that more than 100 disqualified ballots likely supporting the Democrat be counted, but the suit was ultimately rejected by a U.S. district court on Thursday.

The Republican-leaning district is considered a pick-up opportunity for the GOP this cycle, particularly after McSally narrowly lost to Barber by less than one point in 2012. She attracted big-name support throughout the race, including earning the first endorsement from John Bolton’s PAC earlier this year. There is currently no timetable for when the recount will be completed.