WASHINGTON — Sen. Cory Booker must raise nearly $2 million in the next 10 days or the presidential candidate has no "legitimate long-term path forward," according to a memo to staff from the campaign manager obtained by NBC News.

The struggling candidate's campaign manager, Addisu Demissie, warned that after weaker-than-expected fundraising in the early part of September, the campaign needs to rake in another $1.7 million before the last day of the financial quarter on Sept. 30.

"Without a fundraising surge to close out this quarter, we do not see a legitimate long-term path forward," Demissie wrote in the Saturday memo to staff and supporters. "The next 10 days will determine whether Cory Booker can stay in this race."

Booker was one of the first candidates to build a substantial operation in critical early states, and his ground game in Iowa has been praised by local activists, but the large headcount is expensive.

Demissie said the campaign has enough money to keep going at its current pace if it wanted to, but not enough to expand operations before voting starts early next year and that Booker doesn't want stay in the race if he doesn't think he can win.

"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," Demissie wrote.

Booker's campaign has struggled nationally. He averages at just under 3 percent support in the polls, according to Real Clear Politics.

He warned other candidates were in similar situations and that the historically large 2020 field "is about to narrow dramatically."

"Booker might not be in this race for much longer — the same is true for other important voices in the field," Demissie wrote.

He argued there probably only four campaigns with the "money necessary to build and sustain the national organization needed to win," presumably referring to former Vice President Joe Biden, Massachusetts Sen. Elizabeth Warren, Vermont Sen. Bernie Sanders and Mayor Pete Buttigieg of South Bend, Indiana, who raised more money last quarter than any other candidate.

Candidates often use sky-is-falling language in fundraising emails, but Demissie insisted that is not the case for Booker — even if he is hoping to use the warning to raise money.

"I want to be clear: This isn't an end-of-quarter stunt or another one of those memos from a campaign trying to spin the press," he wrote. "This is a real, unvarnished look under the hood of our operation at a level of transparency unprecedented in modern presidential campaigns."

Booker and the rest of the Democratic field will address Iowa Democrats at the Polk County Steak Fry Saturday as some candidates have begun taking more drastic action to try to shake up what has been a surprisingly static race so far.

Starting there, Booker and his campaign will try to remind Democrats why they made the former Newark mayor a national star in the first place and what they might miss if he were to exit the race, such as his powerful, optimistic oratory and roots in a poor inner-city neighborhood, according to aides.

"The fourth quarter's when you gotta grow," Booker told MSNBC's Vaughn Hillyard at Steak Fry. "Grow or get out and we are not in this for an ego exercise, this is not a vanity play. I'm in this to win the nomination and beat Donald Trump. And so if you believe in my campaign, if you believe in my voice being on this stage, this is a determining moment for us, we either are going to reach a 1.7 million dollars to stay in this race or we're gonna have to make really tough decisions at the end of this quarter."

And in a bid to sell transparency, the campaign will keep a constantly updating tracker of their progress toward the $1.7 million goal.

Booker is a crowd favorite at candidate events and he's on many voters list of potential favorites to support, even though he occupies the top slot for a much smaller number. But his campaign will lean into that, urging rank-and-file Democrats to chip into to keep him in the race even if they're not planning to vote for him.

Minyon Moore, a veteran Democratic strategist who is neutral in the 2020 race and has donated to multiple candidates, said while she understands voters' fatigue with the massive field, Booker is a valuable addition.

"I think that it would be a travesty not to have him in the race," she said. "We need him in the race. His race is one factor, but his voice is another factor."

Money is typically what forces candidates out of a presidential contest. New York City Mayor Bill de Blasio on Friday became the latest candidate to quit the race after realizing he had no chance of making the October debate stage.