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WASHINGTON (Reuters) - New Jersey Senator Cory Booker raised more than $5 million for his presidential election campaign in February and March, and has more than $6.1 million in cash on hand, his campaign said on Sunday.

That is considerably less than others among the more than 15 Democrats who have announced they are running for the party’s nomination.

U.S. Senator Kamala Harris of California raised $12 million in the first three months of 2019 while Pete Buttigieg, the mayor of South Bend, Indiana, who recently saw a bump in opinion polls but is still considered a long-shot, announced Monday that he had raised $7 million.

Beto O’Rourke, a former U.S congressman from Texas, raised $9.4 million in the first 18 days of his bid for the presidency, his campaign said. U.S. Senator Bernie Sanders raised $5.9 million in the first day after announcing his candidacy, and later disclosed he had raised $10 million in a week.

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

Donations to Booker averaged about $34, with 82 percent of the donors new supporters of the candidate, spokeswoman Sabrina Singh said on Twitter.

Candidates are required by law to report all campaign donations, and cannot accept more than $2,800 from a single donor during the primary race.

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