WASHINGTON (Reuters) - Senator Elizabeth Warren issued a pointed campaign finance challenge to fellow Democratic U.S. presidential contenders at her official 2020 White House election launch.

FILE PHOTO: U.S. Senator Elizabeth Warren (D-MA) speaks at a rally to launch her campaign for the 2020 Democratic presidential nomination in Lawrence, Massachusetts, U.S., February 9, 2019. REUTERS/Brian Snyder/File Photo

“I’m not taking a dime of PAC money in this campaign. I’m not taking a single check from a federal lobbyist. I’m not taking applications from billionaires who want to run a Super PAC on my behalf. And I challenge every other candidate who asks for your vote in this primary to say exactly the same thing,” Warren said during her rally last Saturday in Massachusetts.

Warren’s move regarding money from lobbyists and political action committees (PACs) - entities formed by corporations, unions and others to raise and spend money to back or oppose candidates - highlights the tricky role money is expected to play in a Democratic primary battle that could draw dozens of candidates vying to challenge Republican President Donald Trump.

The Democratic candidates are expected to push one another to show their commitment to getting big money out of politics, following the lead of Democrats who helped the party retake control of the U.S. House of Representatives from Trump’s fellow Republicans in last year’s congressional elections.

Many Democrats have decried the influence of money from corporations and wealthy individuals on elections. But some strategists worry that the candidates could go too far and hamper the party’s chances in the November 2020 general election by giving a financial edge to the Republicans.

All of the Democrats who have launched a 2020 campaign or are formally considering a run have sworn off accepting corporate PAC money, campaign contributions that are pooled by corporate fundraising committees.

Some of the 2020 contenders are taking the pledge a step further by promising to reject donations from registered lobbyists and other PACs, while discouraging support from some types of so-called Super PACs that can raise and spend unlimited sums.

Senators Kamala Harris of California, Cory Booker of New Jersey, Kirsten Gillibrand of New York, Amy Klobuchar of Minnesota and Warren of Massachusetts will not accept donations from registered lobbyists, according to the candidates and campaign aides. The campaign of former U.S. Housing and Urban Development Secretary Julian Castro, a Texan, already has returned money to several lobbyists, an aide told Reuters.

Most of the candidates, including Klobuchar, Harris, Booker, Gillibrand, Warren and Castro, have said they also will discourage single-candidate Super PACs from operating on their behalf, though they cannot prevent them from doing so.

During Booker’s first campaign swing through Iowa, the state that holds the first party nominating contest in early 2020, he said he was frustrated that a donor had started a Super PAC on his behalf, and he did not want its support.

“My campaign is going to be run by the people,” Booker said. “We’re going to power this election principally by low-dollar contributions.”

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‘HAND-WRINGING IN WASHINGTON’

Legally, Super PACs are supposed to operate independently of candidates and cannot contribute to them directly. In reality, they often are run by donors close to the candidates and their campaigns.

David Donnelly, president of Every Voice, a campaign finance watchdog group, said candidates are smart to “think twice about winking and nodding at the Super PACs that are set up for themselves.” They “may be more of an albatross than a benefit” with Democratic primary voters, Donnelly said.

Promising to reject all Super PAC support, however, could cause consternation. Super PACs supporting Trump, the presumed Republican nominee, have continued raising money throughout his presidency.

Donnelly said that “you’d see a lot of hand-wringing in Washington about a candidate” who would forsake any Super PAC support, saying such a stance could be read as a “signal you’re not serious about winning.”

Money is spent by the millions of dollars in presidential races. With donations to candidates from PACs capped at $5,000, these entities play an insignificant role in these elections.

In contrast, Priorities USA Action, which is the largest Democratic Super PAC and is not tied to a specific candidate, spent more than $190 million to help Democrat Hillary Clinton’s 2016 presidential campaign, according to data from the Center for Responsive Politics, which tracks money in politics.

It is not yet clear if a Democrat would discourage the support of Super PACs like Priorities once becoming the party’s nominee. Priorities is remaining neutral in the primary battle but likely will have hundreds of millions of dollars at its disposal for the general election.

Gillibrand told reporters she did not think individual candidates should have Super PACs but did not weigh in on Super PAC support more generally. The Harris campaign told Reuters she “rejects Super PAC activity” but did not specify whether that extended to groups like Priorities.

The campaigns of Warren, Booker and Klobuchar likewise did not respond directly to questions from Reuters about how they would handle support from such entities. Castro’s campaign said he would discourage all Super PAC activity.

Foregoing big money groups will put more focus on a Democratic candidate’s ability to attract large numbers of small-dollar donations.

Klobuchar’s campaign said it raised more than $1 million from “online and grassroots supporters across the country” in the 48 hours after she announced her candidacy on Sunday.

Supporters who heeded Klobuchar’s call to “text Amy” received this reply: “This team is powered by YOU (i.e. NOT Super PACs). Chip in now to help support this movement.”