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

Iowa senior adviser Joe O’Hern confirmed to The Associated Press on Monday that he had stepped down from 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.

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While O’Hern was a key staffer on Booker’s Iowa team, Booker’s campaign has long been seen as one of the strongest on the ground in Iowa, a state that remains a top priority for his presidential campaign.

The candidate hired some of the state’s top political staffers and started organizing earlier than many of the top-polling candidates in the state. But he’s struggled to gain traction in both Iowa and national polls.

A recent USA Today/Suffolk University survey of likely Iowa caucusgoers — the only one taken since the first Democratic primary debate — found Booker polling at 2% support in the state. Nationally, he fares about the same in recent polling.

That does mean Booker has met one qualification to appear at the September DNC-hosted primary debates — drawing 2% support in four qualifying national polls. But he has yet to hit the 130,000 unique donations that candidates need to fully qualify for the debate. Booker said a week ago his campaign was just 15,000 donors short of that mark.

While his campaign saw a slight drop-off in fundraising from his first quarter in the race, it announced Monday that it had raised $4.5 million in the second quarter, with $5.4 million cash on hand.