Sen. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.) is fundraising from his fellow 2020 candidate, former Rep. Beto O'Rourke Beto O'RourkeJimmy Carter says his son smoked pot with Willie Nelson on White House roof O'Rourke endorses Kennedy for Senate: 'A champion for the values we're most proud of' 2020 Democrats do convention Zoom call MORE (D-Texas), raising $6.1 million on the first day of his campaign.

"Here is the truth: this morning, Beto O’Rourke announced that he raised more money than us on the first day of his presidential campaign," Sanders said in an email sent to supporters Monday obtained by The Hill. "The good news is, we more than likely had a lot more individual donations than he did."

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O'Rourke, who said Saturday that he was not planning any large-scale fundraisers and that he would not accept any lobbyist or PAC money, did not disclose information such as the number of donors or average contribution size.

Sanders, who announced his second bid for president late last month, raised roughly $5.9 million in the first 24 hours of his campaign.

Sanders disclosed that the average contribution on the first day was just $27 and that his campaign had more than 225,000 unique donors.

O'Rourke and Sanders raised significantly more in the first 24 hours of their campaigns than other 2020 Democratic hopefuls.

Sen. Kamala Harris Kamala HarrisButtigieg stands in as Pence for Harris's debate practice First presidential debate to cover coronavirus, Supreme Court Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (D-Calif.) raised $1.5 million in the first 24 hours after announcing her candidacy last month.

Sen. Amy Klobuchar Amy KlobucharSocial media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Minn.) raised more than $1 million in the 48 hours after she formally launched her presidential bid, her campaign said. Former Colorado Gov. John Hickenlooper announced a similar fundraising total.

Sen. Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE (D-Mass.) raised roughly $300,000 on New Year's Eve, the day she formed an exploratory committee, according to filings from the online donation clearinghouse ActBlue. Her campaign has declined to disclose their complete one-day fundraising totals.

Washington Gov. Jay Inslee broke $1 million after 72 hours.

Other candidates have not shared their first day totals.