WASHINGTON (CNN) Just five candidates accounted for roughly three-quarters of all the money raised in the second quarter of this year by Democratic presidential contenders -- as a crowded field broke into two distinct categories: the haves and the have-nots.

Leading those flush with cash: Pete Buttigieg , a mayor little-known outside his home state of Indiana before his sure-footed media appearances in the spring and cross-country fundraising tours persuaded deep-pocketed Democratic donors to open their wallets.

Buttigieg raised nearly $25 million in the April-to-June fundraising quarter. And he entered July -- typically a slow fundraising period as donors turn to vacation plans and away from politics -- with more than $22.7 million remaining in his bank account, one of the field's biggest war chests. He's already using the money to build a ground force of more than 250 staffers in early voting states

The other four candidates in the top fundraising tier: former Vice President Joe Biden , who collected nearly $22 million, Massachusetts Sen. Elizabeth Warren at $19.2 million, Vermont Sen. Bernie Sanders at $18 million and California Sen. Kamala Harris with $11.8 million

But the vast majority of the field failed to break the $5 million mark during the three-month period, newly filed Federal Election Commission reports show.

That spells trouble for Democrats hoping to compete next year in an Iowa caucus system that requires money and organization months in advance, said Ami Copeland, a former deputy finance director for the Obama campaign.

"When you are running a campaign based on $2.8 million or $4 million and are going up against a $15 million to $20 million juggernaut, it's just hard to penetrate," he told CNN. "The demands of a campaign operation are way too great today."

"We're going to see more candidates drop out before the end of the next quarter," Copeland added.

O'Rourke sputters

He also spent far more than he took in, plowing more than $5.3 million into his political operation. About $1 in $5 of O'Rourke's dollars went to payroll costs.

In an email to supporters Monday night, O'Rourke campaign manager Jen O'Malley Dillon bluntly acknowledged that fundraising needed to increase.

"We have the resources we need to be in this for the long haul, but if we don't raise more this quarter, we'll have no choice but to make some adjustments," O'Malley Dillon wrote.

Payroll, planes and celebrities

Sanders outspent the rest of the Democratic field over the three-month period, spending roughly $14 million -- including $3 million directed to salaries.

Biden, running in his first presidential race since 2008, racked up $11 million in expenses, including more than $250,000 with a private jet operator based in Arlington, Virginia.

But the glitterati have found their way into her campaign. Hollywood A-listers who contributed the maximum $2,800 primary donation to Warren included TV drama guru Shonda Rhimes and actress Jane Fonda. Barbra Streisand donated $1,000 to the Massachusetts senator.

Even so, small donations of $200 or less still powered Warren's campaign, accounting for two-thirds of her individual contributions between April and June. Another candidate, Julian Castro , raised even a greater share of his money from small donors, a little more than 74%.

Trump juggernaut

President Donald Trump built his 2016 campaign on small donors and still spends heavily to reach his grassroots base. But his campaign and joint fundraising committees he operates with the Republican National Committee also increasingly are relying on big donors to fuel his re-election.

Together, four related Trump entities -- including the national party -- announced raising a combined $108 million during the second quarter.

Donors who contributed $360,600 apiece to his Trump Victory fund included Wisconsin roofing magnate Diane Hendricks; Marlene Ricketts, the wife of TD Ameritrade founder J. Joe Rickett; and Linda McMahon, who served as Trump's Small Business Administration chief before leaving to raise money for a pro-Trump super PAC.

Trump Victory shares its money with the President's campaign, the RNC and state parties.

Freshman fever

Some freshman Democrats in Congress are raising substantial sums, newly filed campaign reports show.

New York Rep. Alexandria Ocasio-Cortez -- at the center of a firestorm over Trump tweets telling her and three other women of color in Congress to "go back" where they came from -- raised a whopping $1.2 million in three months as she looks ahead to reelection in 2020.

It's an amount that eclipsed the second-quarter hauls of some Democratic presidential candidates including New York City Mayor Bill de Blasio and former Colorado Gov. John Hickenlooper.

Nearly 84% of her donations came in small amounts, a sign that she can tap those donors repeatedly before hitting the $2,800 limit on what an individual can donate to a primary or general election.

Burn rates

The filings show that 11 Democratic candidates -- all outside the top five -- posted unsustainable burn rates, as they spent more than they raised in the second quarter.

Highest among those was New York Sen. Kirsten Gillibrand with a burn rate of nearly 184% as she spent $4.2 million. Gillibrand has a sizable $8.2 million cash-on-hand reserve, padded by pre-existing funds from her Senate campaign account. But with only $2.3 million raised in the quarter, the campaign can't keep spending at that rate.

Candidates like Booker and Klobuchar have large enough cash reserves to weather the heavy spending. But many candidates, even those who didn't spend more than they raised, are entering the third quarter with limited resources. Eight candidates posted cash on hand totals of less than $1 million, including Ohio Rep. Tim Ryan , Massachusetts Rep. Seth Moulton , New York City Mayor Bill de Blasio and businessman Andrew Yang

This story has been updated.

CORRECTION: This story has been updated to accurately describe Andrew Yang as a businessman.