But several are going beyond that, responding to demands that they spurn outside assistance from independent groups or cease accepting donations from employees of specific companies, among other strictures. The fiercest battle so far has been over whether candidates should accept money from those employed in the oil and gas industry — one seen as acting contrary to the party’s position on climate change.

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Sen. Elizabeth Warren (D-Mass.) is poised to make self-imposed limits a key part of a campaign, pledging as she did in her recent statewide race to refuse contributions from corporate PACs or from registered federal lobbyists. She also plans to avoid backing from a super PAC — and would be unlikely to endorse any candidate who relies on a super PAC, according to a person familiar with her thinking.

Rep. Beto O’Rourke (D-Tex.) made campaign finance issues a centerpiece of his unsuccessful bid for U.S. Senate in Texas, a race in which he raised record small-dollar donations while disavowing donations from any PAC.

Sen. Cory Booker (D-N.J.), on the other hand, has significant ties to Wall Street and Silicon Valley and in past campaigns has raised the bulk of his money from major donors. A super PAC already has formed to back him, an alliance that his campaign may calculate it needs but could become a political liability among Democrats seeking a demonstration of grass-roots support over multimillionaire backers.

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Those considering seeking the Democratic nomination see their decisions about how to raise money as definitional to voters — even if those choices may also put them at a disadvantage against a deep-pocketed President Trump, whose entwined campaign and Republican Party apparatus have broken records for fundraising in the past.

“This is going to be a defining issue for Democrats, and it should be,” said Robert Zimmerman, a prominent fundraiser and Democratic National Committee member from New York. “It’s not acceptable for Democrats to talk about campaign finance reform and then volunteer to be part of the corrupt system that they’re condemning.”

One of the first indications of political potency for the 2020 candidates will be how much each can raise in the first quarter of 2019. That is one reason several prominent candidates are expected to announce their campaigns in early January, giving them maximum time to raise as much money as possible before the end of March.

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Several billionaires — the richest of them being Michael Bloomberg, the businessman and former mayor of New York — are likely to run, giving them immediate access to resources and putting pressure on the rest of the field. The billionaires will not face as much demand to raise money — which could insulate them from attacks on the sources of outside funds — but they also could face criticism of their extreme wealth in an increasingly populist party.

Several potential candidates, among them Warren, O’Rourke and Sen. Kamala D. Harris (D-Calif.), would enter the contest with another kind of advantage: formidable donor email lists they have built during past campaigns, which conceivably could place them atop the non-millionaire flank of candidates. But campaign aides privately concede that there is probably a large overlap among donors on those lists. Some could choose to donate to several candidates, but more likely they will either pick one or wait for the field to settle out.

The Democratic fundraising landscape has changed dramatically from four years ago, when Hillary Clinton won the nomination after amassing a large, moneyed donor base. The party’s activists have moved more in the direction of Sen. Bernie Sanders (I-Vt.), who led a campaign fueled by small-dollar donors in which he unleashed a barrage of damaging criticism of Clinton’s ties to wealthy benefactors. (His regular bragging about not having a super PAC became a “Saturday Night Live” punchline, even if Sanders did benefit from a super PAC run by a nurses union.)

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“I absolutely think it’s the way campaigns distinguish themselves and send a message about whose voice is more important,” said Jeff Weaver, a top strategist for Sanders. “A campaign that relies on a super PAC as a principal source of funding has to question whether there’s the grass-roots level of support they need to go forward.”

On that front, there are substantial differences among the candidates, if past races are any indication. Since 2013, Warren received 56 percent of her donations — and Sanders 76 percent of his — from donors giving less than $200, according to the Center for Responsive Politics. Less than 12 percent of Booker’s donations have come from those small-dollar donors, the center found. About a quarter of the donations to Sens. Amy Klobuchar (D-Minn.) and Sherrod Brown (D-Ohio), also potential candidates, were from small-dollar donors, compared with about a third of those to Harris and Sen. Kirsten Gillibrand (D-N.Y.).

An example of the pressure on would-be candidates came earlier this year, when Harris was asked during a town hall in Sacramento if she would turn down donations offered by a corporation or a corporate lobbyist.

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“Well, it depends. It depends,” Harris said.

“Wrong answer,” the questioner responded.

“Well, that’s not the answer you want to hear,” Harris said. “It doesn’t make it wrong.”

Within a few weeks, Harris reversed herself, saying that after reflecting on the town hall exchange, she decided she would join other politicians in pledging not to take PAC money from corporations.

The pledge not to take corporate PAC money — also made by Booker and Gillibrand earlier in the year — has as much a symbolic meaning as a financial one. Corporate PAC donations have not played a significant role in the financing of campaigns, and candidates who won’t take the PAC money from corporate interests often still take individual donations from corporate executives or lobbyists.

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That has led some candidates to go further to demonstrate fealty to changes in the campaign finance system.

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O’Rourke, during his Senate campaign, refused to take money from any PAC, even those not tied to corporations. (This was a shift; during his House races in 2012 and 2014 he took PAC money).

“Political action committees represent the corporations and interests that have business before Congress — so the pharmaceutical industries, the telecom industries, the energy industries, the insurance industries — and they give money to members of Congress not just for access, although that’s part of it,” O’Rourke said on Bill Maher’s HBO show in March. “They’re also buying outcomes, actual legislative language that appears in the bills, and in the bills that become laws.”

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Warren has been one of the most outspoken candidates when it comes to political spending, filing legislation to require more disclosures for corporations and super PACs. A bill she filed in August would ban lobbyists from donating to anyone holding or running for federal office.

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“The recent explosion of big political spending has delivered a gut punch to our democracy,” Warren said in a recent speech. “There’s a lot of work to do on campaign finance, starting with overturning Citizens United. But that’s not nearly enough.”

Warren just won an uncompetitive reelection campaign and has $12.5 million on hand, which would make it easier to forsake any outside help.

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Booker’s and Harris’s calculations are different, and influenced by their supporters. A super PAC benefiting Booker could draw from Wall Street money, while Harris would have wealthy donors in California ready to support her campaign.

The outside effort aimed at helping Booker is being organized by Steve Phillips, a San Francisco-based Democratic donor. As first reported by the New York Times, he plans to raise $10 million for the effort, to be called Dream ­United.

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Super PACs are prohibited from coordinating with a candidate but can raise unlimited sums from donors. Campaign donations from individual donors are limited to $2,700 per election.

Beyond the divisions over outside groups, Democrats are also tussling over whether to invoke a purity test regarding specific ­donors.

Leading up to November’s midterm election, Oil Change USA, a coalition of environmental groups, tried to get candidates to sign a “No Fossil Fuel Money Pledge” to not accept contributions over $200 from PACs or executives of fossil fuel companies. They have met with various presidential contenders — including Warren, Booker and Harris — but so far only Sanders and Sen. Jeff Merkley (D-Ore.) have agreed.

The group plans to send people to candidate events to try to pressure them into either signing the pledge or defending their reason for taking fossil fuel money.

A preview came in the recent U.S. Senate race in Texas. An activist affiliated with the effort persuaded O’Rourke to sign what appeared to be a copy of the pledge during his campaign against incumbent Republican Sen. Ted Cruz. Then, however, O’Rourke took money from oil and gas executives.

O’Rourke said he thought the pledge only covered not taking PAC money and that the group did not make clear to him that the pledge also included not taking money from those who work in the industry. The group recently removed O’Rourke from the list of those who had taken the pledge.

“It’s certainly fair to highlight the fact that he has taken that money from the industry,” said David Turnbull, a spokesman for Oil Change USA. “My concern is making sure we don’t misrepresent what he believed to have signed.”

Candidates trying to navigate the changing mores also are encountering a more basic problem: The size of the potential field has made it difficult to get financial traction behind the scenes.

“The candidates aren’t committed to running; the donors aren’t committing to candidates,” said Zimmerman, the Democratic donor. “It’s like an eighth-grade dance. Everybody is afraid to make the first move.”

Ellen Susman, a Democratic donor in Texas, has gotten calls from several prospective candidates and has met with a few of them. But she and her husband won’t give until they can figure out whom to back — “we will not play the support-them-all game and see who wins” — and at the moment, she is undecided.