Washington (CNN) Denouncing big money in politics is fast becoming a rallying cry for Democrats hoping to win over the party's progressive voters ahead of the 2020 presidential election.

But some of the party's fundraisers and strategists say those candidates may have little power to rein in the party's big elite fundraisers and donors, who are eager to raise and spend whatever it takes to topple President Donald Trump.

Swearing off super PACs headed into 2020 is "like bringing a butter knife to a gun fight," said Dallas lawyer Marc Stanley, who raised money for President Barack Obama and 2016 Democratic nominee Hillary Clinton. Stanley also helped create a super PAC that attacked Texas GOP Sen. Ted Cruz on the airwaves during the 2018 Senate campaign, despite Democratic nominee Beto O'Rourke disavowing such groups.

"I would prefer that we had public funding of campaigns or just small donors funding campaigns," Stanley told CNN this week. "But it's just not reality. We need to be prepared to fight. The consequences of this election are just too great."

A number of likely candidates are renouncing super PACs and direct donations from corporate political actions committees. For example, former House and Urban Development Secretary Julian Castro, who is scheduled to announce his 2020 plans on Saturday, recently pledged to refuse PAC contributions and has challenged other Democrats to do the same.

O' Rourke, who is mulling his own bid, rejected all PAC money in his unsuccessful Senate campaign and still raised a staggering $80 million, powered, in part, by his online supporters. This week, aides to Massachusetts Sen. Elizabeth Warren, who recently kicked off a presidential exploratory committee , upped the demand that Democrats reject super PACs and self-funding candidates, blasting out a campaign email that urged rivals "to show some moral backbone."

Early super PAC activity

But some Democratic super PACs already have begun to amass large sums to prepare for a protracted battle for the party's nomination and to confront a President who raised an unprecedented $106 million for his re-election before even hitting the two-year mark of his first term.

Steve Phillips, a prominent San Francisco donor, said he has secured $4 million in pledges for Dream United, a super PAC he recently established to support a potential presidential bid by New Jersey Democratic Sen. Cory Booker. He said he's hopeful he can raise $10 million by the end of March, the first fundraising quarter of the election cycle.

Super PACs are a powerful weapon in politics because they allow political operatives to raise and spend unlimited amounts of money from virtually any source as long as they do not coordinate their spending decisions with candidates.

Phillips himself staged an independent effort that spent $11 million to back Obama's first presidential campaign in 2008, even though Obama never endorsed the move. He said his new group will provide a vehicle for well-off donors, particularly those from racial and ethnic minority groups, to show their support for Booker, the first African-American elected to the US Senate from New Jersey.

"Cory, like Obama, inspires people across the full economic spectrum," Phillips said. He said Booker built relationships with low-income New Jersey residents during his tenure as Newark mayor but also has "enthusiastic supporters in the 1%."

Booker has not endorsed Phillips' effort. Booker spokesman Jeff Giertz told CNN that the senator still is weighing whether to seek the presidency.

"There has been no activity on his part or that of his team to organize or endorse the creation of a super PAC," Giertz said in an email.

But Phillips said his work will continue, likely focused on promoting Booker's candidacy in the states that follow Iowa and New Hampshire in the nomination process, including South Carolina, Nevada and possibly California, which recently moved up its primary to March.

Another super PAC, Priorities USA Action, and its nonprofit arm already are working to build an operation to target Trump and defend Democrats' eventual nominee. Priorities, created to help Obama win a second term in 2012 and deployed again to aid Hillary Clinton in 2016, has secured pledges totaling $74 million, said its executive director Patrick McHugh. That's nearly twice what it had raised at the same point in the last presidential cycle, he said.

"We agree that Citizens United should be overturned to get big money out of politics," McHugh said, referring to Supreme Court ruling that helped pave the way for unlimited corporate spending in candidate races. "But we already are facing an onslaught of money from the other side, and we've got be prepared for the general election when it comes."

Self-funders

Warren, a Harvard professor turned consumer advocate, already has a vast base of small-dollar donors. More than $6 out of every $10 she collected from individuals for her 2018 Senate re-election came in amounts of $200 or less -- one of the highest proportions of small-donor support among members of Congress considering White House campaigns, federal campaign-finance records show.

Within days of announcing her exploratory committee, Warren fired warning shots at moneyed interests , criticizing both super PACs and the wealthy businessmen who are considering funding their own campaigns for the Democratic nomination.

"No to the billionaires," Warren said earlier this month on MSNBC's "The Rachel Maddow Show," asking Democrats to "link arms" in support of grass-roots funding.

This week, one billionaire -- former hedge-fund manager Tom Steyer -- took himself out of the running for president. But others, most notably former New York mayor Michael Bloomberg, are considering bids.

Bloomberg, speaking Friday in Texas, said using his own money to fund his three successful campaigns for New York mayor guaranteed he was not beholden to contributors. He said he would self-fund a White House bid if he runs.

"I think not having to adjust what you say and what you worked on, based on who financed your campaign, is one of the things that the public really likes," Bloomberg said.

Another independently wealthy businessman, former Maryland congressman John Delaney, has been running for president for nearly two years, primarily off money he has personally given to his long-shot campaign.

"I don't think it is a negative at all to invest your money in yourself and campaign," said Delaney, who also raised money for Clinton in 2016. "I have more respect for the American people than that and I think they, when you are running for President, the candidates who are good, whether they are self-funders or not, will rise to the top."

In an interview with CNN, Delaney acknowledged that the rise of digital fundraising makes large-dollar donations less important.

"It has to be that. The question is to what degree," he said. "The fact there is a whole new way for people to raise money, by definition, means that traditional fundraising is a smaller part of the pie. But that doesn't mean it's not important."

Steve Elmendorf, a veteran Washington lobbyist who raised money for Clinton and served as a senior aide in Democrat John Kerry's 2004 presidential campaign, said pragmatism, rather than lofty principles, likely will govern how much candidates rely on super PACs and the fundraisers who "bundle" together campaign donations.

"If you are a senator or a governor who has no proven record of raising online money, you're going to need some bundlers to get you going on the ground in Iowa, New Hampshire, South Carolina and Nevada."

"Not to be cynical, but I think people adjust to the situation at hand," Elmendorf told CNN. "Elizabeth Warren doesn't need a super PAC. She's a proven online fundraiser. Some other people may."