• Kamala Harris is already seeing a wave of big money donor support since President Joe Biden dropped out of the race.
• Some of the party's top financiers opted to hold back funds after Biden's June debate performance drew concerns from Democrats about his mental acuity.
• The progressive donation platform ActBlue said it raised $46.7 million after Biden endorsed Harris.

Vice President Kamala Harris is already seeing a wave of big money donor support following President Joe Biden dropping out of the race, with the help of bundlers who have assisted her in past races, according to people familiar with the matter.

Moments after Biden announced he would drop out of the race for president and endorsed Harris, major donors who helped raise money during her failed 2020 Democratic primary fight and successful 2016 Senate campaign immediately mobilized and began outreach to wealthy contributors, according to these people, who spoke to CNBC did so on the condition of anonymity in order to speak freely.

"I've been in touch with many of her supporters from New York to California and we are getting organized to start up the fundraising machine," said Jon Henes, the vice president's national finance chair during her 2020 run for president. "I had more than 200 texts, calls, and emails today of people wanting to host events and make donations."

Among the top party fundraisers now planning to help Harris is longtime Wall Street executive Marc Lasry, according to a person with direct knowledge of the matter. Lasry helped raise money for the vice president when she ran against Biden in the 2020 Democratic primary. He later raised campaign cash for Biden when he beat Trump during that election cycle

But it wasn't just the big donors who responded to Biden's announcement: The progressive donation platform ActBlue initially said it raised $27.5 million from small-dollar donors in the five hours after Biden endorsed Harris. Later, the company announced it raised over $45 million.

Shortly after the endorsement, the Biden for President campaign filed Federal Election Commission paperwork to change its name to "Harris for President."

But the renewed energy among some of the party's top financiers was particularly significant because several of them had announced they would hold back funds from a Biden-led ticket, after his disastrous debate performance on June 27 against former President Donald Trump.

Venture capitalist Reid Hoffman plans to donate even more in support of Harris' candidacy than he did when he backed Biden over Trump throughout the 2024 presidential election cycle, according to a person familiar with the matter. Hoffman has donated at least $10 million to supportive Biden political action committees so far this election, according to Federal Election Commission records.

Hoffman endorsed Harris in a social media post. Hoffman's spokeswoman declined to comment.

Before Biden dropped out, Harris' allies were planning a fundraiser in the ritzy New York Hamptons region for early August, which was set to feature Second Gentleman Doug Emhoff, some of the people explained. Some donors were deleting the emailed invite out of frustration with Biden, these people explained.

Now, the expectation is that a Hamptons event next month will likely sell out with many donors getting off the sidelines to help Harris, according to a person familiar with the event.

Alexander Soros, the son of Democratic megadonor George Soros, said in a social media post on Sunday after Biden dropped out that "it's time for us all to unite around Kamala Harris and beat Donald Trump."

But the major donor support for Harris isn't universal across the Democratic Party, suggesting she has some work to do in order to get them on her side.

Stewart Bainum Jr., the chairman of Choice Hotels and a major Biden donor, told CNBC he is still sitting on the sidelines and won't jump in to help Harris because he feels there should be a battle at the upcoming convention with other potential nominees having the chance to lead the party. The Democratic National Convention is set to take place in Chicago in August.

Another veteran Biden fundraiser said there should be a "mini primary" over the coming weeks heading into the convention for Harris and anyone else who wants to compete for the nomination.

But for Harris, having some donors immediately jump on board is a sign that she could bolster the over $95 million campaign war chest she'll control after Biden departed from the race.

Harris can likely get immediate access to the Biden campaign's roughly $96 million donation pot, according to Anna Massoglia, an investigations manager at the campaign finance research center, OpenSecrets.

"The general consensus among most people that I've spoken with is that she can use the funding," Massoglia told CNBC in an interview.

Massoglia noted that some conservative election lawyers believe Harris needs to officially become the Democratic nominee before getting full access to the campaign war chest, though she added that they represent "a very small sliver" of lawyers' opinions.

Still, until election lawyers resolve the debate over whether Harris needs to be the official nominee, her campaign funding access is still an open question. The same goes for the donations of the Democratic National Committee and joint fundraising committees, Massoglia said.

The scenarios get more tangled, however, if Harris does not secure the Democratic nomination. In that case, the campaign could convert its funds into a PAC or another type of political committee that would spend in support of the new nominee.

That option comes with several disadvantages, Massoglia said: PACs are subject to more expensive advertising rates and are not allowed to coordinate with the candidate.

An alternative would be to refund donors and ask them to contribute to another committee. That would come with the added risk of donors potentially deciding not to donate to the new campaign, Massoglia said.