Salary expenses for Cory Booker's presidential campaign were among the highest in the Democratic primary field, but his fundraising haul fell short of his top competitors, raising questions about the New Jersey senator's lasting power in the race.

Booker's second quarter campaign disclosure shows he disbursed nearly $1.6 million in salary payments to his staff of more than 150 individuals. The payments topped those of some of his chief rivals for the nomination, such as South Bend mayor Pete Buttigieg and Sen. Kamala Harris (D., Calif.), each of whom spent about $1.4 million on salary, according to an analysis by the Washington Free Beacon.

The Booker campaign's total fundraising haul of $4.5 million, however, was well below Buttigieg, who raised $24.9 million, and Harris, who raised $11.8 million.

The only candidates that outspent Booker on campaign staff were former Vice President Joe Biden, Vermont senator Bernie Sanders, and Massachusetts senator Elizabeth Warren, who are all outpacing Booker both in fundraising and the polls. Booker has failed to register above 2 percent in any major poll conducted in July.

Booker's spending has outpaced his ability to raise money in a crowded Democratic primary field. The campaign spent a total of $5.3 million last quarter, about $800,000 more than he raised, with 30 percent of his spending allocated to staff salaries. He entered July with nearly $5.4 million in cash on hand, which puts him far behind current frontrunners.

The release of Booker's disappointing second quarter disclosure coincided with news that a key member of his staff, Iowa senior adviser Joe O'Hern, was leaving the campaign, citing personal reasons. His campaign did not respond to an inquiry on whether the departure was related to the campaign's money trouble.

Booker's over-extension on salary could spell trouble for his campaign, which has failed to gain traction since it launched in February. Veteran campaign strategist Jim Messina, who ran both of Barack Obama's successful presidential campaigns, predicted on Tuesday that candidates will start dropping out of the 2020 race because of money problems.

"Money in a presidential race is oxygen, and they're going to run out of oxygen if they can't stay competitive," Messina said during an appearance on MSNBC.

Among the other candidates who spent more than they raised last quarter were Sen. Amy Klobuchar (D., Minn.), former Texas congressman Robert "Beto" O'Rourke, and Sen. Kirsten Gillibrand (D., N.Y.), who has more than $8 million left in her campaign account despite spending nearly twice the $2.3 million she raised. None of the three candidates are spending as much as Booker on campaign staff.