Senator Elizabeth Warren (D-MA), a 2020 US Presidential hopeful, speaks during the 'We The People' Summit at the Warner Theatre April 1, 2019, in Washington, DC.

Sen. Elizabeth Warren's presidential campaign said on Wednesday that it raised $6 million from 135,000 donors in the first quarter, a haul that falls behind those of her major rivals.

The announcement comes amid reports that the progressive Massachusetts Democrat is struggling to raise money after shunning traditional sources of big dollar donations.

The average donation was $28, Warren's campaign said. In total, the candidate received 213,000 donations. The campaign said that 99% of donations were $200 or less.

The $6 million, raised between her launch at the start of the year and April, puts Warren behind at least four of her fellow Democrats, despite being the first major candidate to enter into the race. That said, Warren does have an advantage over some in the race in that she transferred $10 million from her campaign for the Senate that she can use for her presidential campaign.

The campaign said that it had $11.2 million in cash on hand, and raised $1.4 million in the final week of the period, including 50,000 new donations.

"I won't sugarcoat it," Roger Lau, Warren's campaign manager, wrote in an email to supporters. "We were outraised by some other candidates in the presidential primary this first quarter."

"Because Elizabeth's been able to count on grassroots donations, she's been able to spend her time visiting as many states as possible, meeting voters, and calling grassroots donors to personally thank them for giving," Lau wrote.

Read: Here's what the 2020 Democratic candidates say they've raised in the first quarter

In comparison to Warren, both Sen. Bernie Sanders, the Vermont independent, and former Texas congressman Beto O'Rourke said they raised about $6 million in the first day of their campaigns. Sanders' average donation for the first quarter was $20, while O'Rourke's was $43.

Official tallies from all the campaigns are due Monday.

The New York Times reported last month that Warren's finance director, Michael Pratt, left the campaign after pleading unsuccessfully in February with the candidate to pursue wealthy donors and not to rule out high-priced fundraisers.

But Warren disagreed. Warren announced later in February that she would not host any events that were limited to those who could write "big checks."

Warren also released her 2018 tax returns on Wednesday. Those showed a family income of slightly more than $900,000.

Other candidates said they raised the following amount in the first quarter:

Sanders raised $18.2 million

Sen. Kamala Harris, D-Calif., raised $12 million

O'Rourke raised $9.4 million

South Bend, Indiana Mayor Pete Buttigieg raised $7 million

Sen. Amy Klobuchar, D-Minn., raised $5.2 million

Sen. Cory Booker, D-N.J., raised $5 million

Andrew Yang, a businessman, raised $1.7 million

-- CNBC's Kevin Breuninger contributed to this report