WASHINGTON (Reuters) - Candidates vying for the Democratic nomination to be U.S. president began on Monday to disclose how much cash they were able to raise in the first quarter of the year - an early test of their ability to organize and build support.

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U.S. Senator Kamala Harris of California raised $12 million in the first three months of 2019, her campaign said late on Monday.

Pete Buttigieg, the mayor of South Bend, Indiana, who recently saw a bump in his standing in public opinion polls but is still considered a long-shot candidate, announced on Monday morning that he raised $7 million in the first quarter.

Other candidates in the race have yet to announce their fundraising numbers, but indications are that some White House hopefuls may post totals easily exceeding those of both Harris and Buttigieg.

When Beto O’Rourke, a former congressman from Texas, entered the race, he raised $6.1 million in a day. That followed U.S. Senator Bernie Sanders, who raised $5.9 million in his first day - and later disclosed he had raised $10 million in a week.

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 deadline will also put a spotlight on those who have had difficulties raising money, including U.S. Senator Kirsten Gillibrand, who has struggled to gain any traction in the polls. Media outlets have reported that she has been less than successful in finding financial backing.

U.S. Senator Elizabeth Warren announced earlier this year that her campaign would not hold any formal fundraising events and instead rely solely on “small-dollar” donations, or contributions collected online.

With such a crowded field - more than 15 Democrats have announced they are running - fundraising abilities have become an early way to prove to donors and potential supporters that a candidate is viable.

Additionally, the Democratic National Committee has said a candidate must have raised money from 65,000 different donors in order to qualify for the first debate to be held on June 26 and 27.