WASHINGTON (Reuters) - U.S. Senator Elizabeth Warren’s presidential campaign raised $6 million in the first quarter of 2019, lagging behind Democratic rivals after swearing off expensive fundraising events, her campaign said on Wednesday.

FILE PHOTO: Democratic 2020 U.S. presidential candidate and U.S. Senator Elizabeth Warren (D-MA) speaks to supporters in Memphis, Tennessee, U.S. March 17, 2019. REUTERS/Karen Pulfer Focht/File Photo

Warren, who had a full three months to fundraise, trails Senators Bernie Sanders, who raised $18 million in six weeks, and Kamala Harris, who raised $12 million in about two months.

Warren’s campaign in an email sent on Wednesday afternoon to supporters acknowledged that she is behind other well-known Democrats, but argued she enjoys grassroots support.

“We don’t have to match other candidates dollar for dollar, but we do need a strong enough grassroots base to be able to keep Elizabeth’s voice front and center in this race,” Roger Lau, Warren’s campaign manager, said in the email to supporters.

The Massachusetts senator announced in February that she would no longer attend expensive fundraising events like dinners and receptions, which may have contributed to her being outraised by some of her rivals. Candidates traditionally use fundraisers to collect large donation checks.

Her campaign said the average donation was $28.

Candidates are required by law to track and report all campaign donations. Donations collected between Jan. 1 and March 31 must be disclosed by April 15. Candidates are limited to collecting $2,800 from a single donor during the primary process.

The April fundraising deadline is one of the first big tests for the more than 15 Democrats vying for their party’s nomination to challenge Republican President Donald Trump in the 2020 election.

Also out-pacing Warren was Beto O’Rourke, the former U.S congressman from Texas who raised $9.4 million in the first 18 days, his campaign said.

Pete Buttigieg, the mayor of South Bend, Indiana, who recently saw a bump in opinion polls but is still considered a long-shot, announced last week that he had raised $7 million during the first quarter.

Warren raised more than two other members of the Senate vying for the nomination. Amy Klobuchar and Cory Booker each raised about $5 million.

Fundraising is an early way to prove to donors and potential supporters that a candidate is viable.

Grassroots fundraising is also one of the qualifying criteria for the Democratic primary debates. Candidates can qualify if they have 65,000 unique donors, along with a minimum of 200 donors per state in at least 20 states.

Warren’s campaign said her donations came from 135,000 individuals.