Cory Booker could be the next Democrat to drop out of the race for the presidential nomination.

In a “now-or-never” memo to supporters and staff, campaign manager Addisu Demissie said Booker need to raise $1.7m by the end of the month or he would have to drop out of the race.

“Without a fundraising surge to close out this quarter, we do not see a legitimate long-term path forward,” Demissie wrote in the memo, published online on Saturday morning. “The next 10 days will determine whether Cory Booker can stay in this race.”

Booker, who is in Iowa this weekend, tweeted: “It’s an unusual move for a campaign like ours to be this transparent, but there can be no courage without vulnerability. I want people to see where we are and understand that we have a pathway to victory, but I can’t walk it alone.”

He insisted the release of the memo was not a “stunt” aimed at boosting fundraising.

“This is a real, unvarnished look under the hood of our campaign at a level of transparency unprecedented in presidential politics,” he wrote.

It would be a shame if that diversity was not reflected in the candidates who end up competing for the nomination Addisu Demissie

Bill de Blasio exited the Democratic race on Friday. Booker has consistently polled better than the New York mayor and made the field of 10 candidates for the debate in Houston last week, which also qualifies him for the debate stage in October.

But despite working assiduously to place operatives, win endorsements and meet voters in Iowa and New Hampshire, key early voting states, he has not been able to keep pace with the leading group in the sprawling field.

Former vice-president Joe Biden, the Massachusetts senator Elizabeth Warren and the Vermont senator Bernie Sanders are the top three, clear in most polls of Pete Buttigieg, the mayor of South Bend, Indiana, and the California senator Kamala Harris.

In the realclearpolitics.com average of polls, Booker also trails tech entrepreneur Andrew Yang and former Texas congressman Beto O’Rourke.

In the memo, Demissie wrote: “If our campaign is not in a financial position to grow, he’s not going to continue to consume resources and attention that can be used to focus on beating Donald Trump, which needs to be everyone’s first priority.

“Booker might not be in this race for much longer – the same is true for other important voices in the field.”

The Minnesota senator Amy Klobuchar trails Booker, followed by the Hawaii representative Tulsi Gabbard, billionaire Tom Steyer, former housing and urban development secretary Julián Castro and the Colorado senator Michael Bennet. Other candidates score even lower.

In a conference call with reporters on Saturday, Demissie said there was a “dissonance” between Booker’s standing in national polls and his support on the ground in early states.

The campaign manager argued that the majority of Democratic voters have not made up their minds and expect that many of the 19 candidates still running will be on the ballot in February.

“People like Cory Booker, they want him in this race,” he said, pushing back on a question about why the candidate has not gained traction.

“The point that we’re trying to make very clearly is the final field that is going to be offered to the Democratic party come February, March and April and beyond is being determined right now here in September,” he said.

Booker is one of two African American senators in the race and part of the most diverse primary field in history. The candidates who have led the field are all white.

“It would be a shame if that diversity was not reflected in the candidates who end up competing for the nomination once people actually start voting come next spring,” Demissie said.

Demissie was adamant the campaign’s “transparency and honesty” would prove that Democratic voters want Booker to stay in the race. But he conceded that if the campaign falls short of its goal, Booker will end his campaign.

On Saturday, Booker and other Democrats were in Iowa for the Polk County Steak Fry.

“If you’re all in for me,” the senator tweeted, “I can’t thank you enough. But if you haven’t settled on a candidate [and] still think my voice belongs in this race, if you believe the Democratic field should include someone like me, I want you to understand the field may narrow [and] pay attention to this too.”