Donald Trump, the presumptive Republican presidential nominee, at the Trump SoHo hotel in New York on Wednesday. (Justin Lane/EPA)

This post has been updated.

Presumptive GOP nominee Donald Trump's call this week for his backers to contribute to his presidential bid generated more than $3 million since Tuesday, according to campaign officials.

"Yesterday my father sent his first ever fundraising email for the campaign and predicted it would shatter fundraising records," read an email to supporters sent Wednesday and signed by his son, Donald J. Trump Jr. "Once again, my father was right. Yesterday's email was a massive success. My father offered to match up to $2 million personally, $1 for every $1 donated, and in less than 12 hours we surpassed that goal. All because of supporters like you."

In an interview with Fox Business Network, national finance chairman Steve Mnuchin said that the campaign brought in more than $3 million online and that Trump would personally match the first $2 million. "So we made over $5 million online yesterday and we’re just starting that effort," he said.

Raising such a huge sum in a short amount of time — through an email list that had never before been tapped for contributions — would make Trump's pitch for donations one of the most successful in American politics, according to veteran fundraisers. Because not every email recipient responds to a solicitation, experts estimate that it would require a list of as many as 6 million supporters to raise a sum as large as 3 million dollars in such a short period. Digital fundraising experts were skeptical that Trump has been able to curate and maintain such a big list. But if he did pull off $3 million in a day, Democrats should be worried, they added. At such a rate, Trump could pull in more than $20 million online by the end of the month.

By comparison, likely Democratic nominee Hillary Clinton raised $28.3 million in May from 430,000 contributors, including $9 million in donations of $200 or less.

Trump's fundraising push came on the heels of a new Federal Election Commission report showing that he raised just $3.1 million in May and entered June with less than $1.3 million in the bank. The paltry sum triggered consternation among Republicans, who are worried that the real estate mogul is not bringing in sufficient funds to compete with Clinton's operation and help the party finance a comprehensive get-out-the-vote effort.

Trump pushed back and accused GOP officials of not rallying around his bid, threatening to self-fund his campaign and leave the party to fend for itself. He also sent out what he declared was his first fundraising email, although an earlier solicitation had gone out under his name from a joint fundraising committee set up in collaboration with the Republican National Committee.

"This is the first fundraising email I have ever sent on behalf of my campaign. That's right. The FIRST ONE," the candidate wrote in the email sent Tuesday. "And, I'm going to help make it the most successful introductory fundraising email in modern political history by personally matching every dollar that comes in WITHIN THE NEXT 48 HOURS, up to $2 million!" 

"Let's make history again, and keep winning, by making this the most successful first fundraising email ever," he added.