DES MOINES, Iowa (AP) — New Jersey Sen. Cory Booker’s Iowa senior adviser is departing his 2020 presidential campaign, leaving Booker without a top staffer in a key early voting state.

Iowa senior adviser Joe O’Hern confirmed to The Associated Press on Monday that he departed the campaign last week for personal reasons. O’Hern was Martin O’Malley’s caucus director in 2016, managed a Democratic gubernatorial primary campaign in 2018 and is widely known among Iowa’s political class as a seasoned organizer with a deep understanding of the caucus process.

Booker’s campaign declined to comment on the departure. Booker has invested considerable time and resources in Iowa, a state that remains a top priority for his presidential campaign. But he’s struggled to gain traction in both Iowa and national polls for the primary.