WASHINGTON (Reuters) - U.S. Democratic presidential candidate Cory Booker will drop out of the running for his party’s 2020 White House nomination unless he can raise $1.7 million over the next 10 days, his campaign said on Saturday.

The mounting scale of rival campaigns and the prospect of higher thresholds for participating in future Democratic debates have forced Booker’s campaign to an “inflection point” where it must grow quickly or have no “legitimate long-term path forward,” according to campaign manager Addisu Demissie.

“If we’re not able to build the campaign organization, which means raise the money that we need to win the nomination, Cory’s not going to continue running and consuming resources that are better used on focusing on beating Donald Trump,” Demissie told reporters on a conference call.

The announcement came as the large, racially diverse Democratic presidential field shows signs of eroding, with fundraising largely dominated by four candidates: former Vice President Joe Biden, Senator Elizabeth Warren, Senator Bernie Sanders and Mayor Pete Buttigieg of South Bend, Indiana.

Booker, a black U.S. senator from Newark, New Jersey, whose support in national opinion polls stands in the low single digits, needs to raise $1.7 million by the time the financial quarter ends on Sept. 30, according to a campaign memo sent to supporters and posted online on Saturday.

“I believe that people will respond,” an optimistic-sounding Booker said in an interview with CNN. “Please don’t look at this as a point where we’re going to say good-bye to Cory. Keep me in this race.”

The debates require a rising threshold of donor numbers and opinion poll support in order for candidates to be allowed to take part.

On Friday, New York City Mayor Bill de Blasio ended his 2020 presidential campaign, leaving 19 other Democrats including Booker to vie for the chance to take on Republican Trump.

Booker and his rival Democratic candidates are due to speak later on Saturday at the Polk County Democrats’ annual Steak Fry in Iowa, which will hold its presidential caucus on Feb. 3.

Demissie told reporters that the funds would be invested in campaign operations in October and November to better ensure success in next year’s early voting states and Super Tuesday primaries.

He dismissed Booker’s weak poll numbers, predicting that the Democrat’s efforts in early voting states would quickly change the dynamics once voters start casting ballots next year, if he could raise the funds necessary to stay in the race.

“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.