• Donors who fund the Biden campaign are using their leverage with congressional leaders to pressure the president to drop out.
• As donors get cold feet, former Secretary of State Hillary Clinton, her husband former President Bill Clinton and Biden campaign co-chair and media mogul Jeff Katzenberg have all made pleas to donors, asking them to stick with Biden.
• Events that feature Vice President Kamala Harris, Biden's likely successor should he step aside, have started to sell out.

On a Tuesday in early July, 75 wealthy Democratic political donors gathered on a Zoom call to discuss the path forward for President Joe Biden after his calamitous debate performance against Donald Trump, according to a person on the call. 

Only one of the donors said they thought Biden should stay in the race, this person said. All the others made it very clear that they believed Biden needed to drop out of the race, if the party wanted to defeat Trump in November. People who spoke to CNBC for this story were granted anonymity to speak freely about a sensitive matter. 

Since then, big money donors who fund either the Biden campaign, his allied political action committees or the party at large have launched a lobbying campaign aimed at senior Democrats in both the House and Senate.

Their goal is to convince lawmakers to publicly call on Biden to end his reelection campaign, according to over half a dozen people familiar with the matter. 

Many of these donors laid their positions out in stark terms: If Biden refused to drop out, they would not be giving money to help his reelection until polls showed that he was a clear favorite to beat Trump. 

Donors who have made these types of calls to Democrats on Capitol Hill include Hollywood executive Ari Emanuel, his brother Zeke Emanuel and Alan Jones, a senior managing director at Intermediate Capital Group and a longtime Democratic party donor, two people familiar with the matter explained.

The effort set off a scramble by some of Biden's allies to keep donors on the president's team.

Former Secretary of State Hillary Clinton, her husband former President Bill Clinton and Biden campaign co-chair and media mogul Jeff Katzenberg have all made pleas to donors, asking them to stick with Biden, according to people briefed on the matter.

Biden has repeatedly said he has no plans to drop out of the race, despite over 20 members of Congress publicly calling on him to "pass the torch," with many more doing so in private. 

"I think almost every donor at this point has publicly or privately expressed their deep concern to the campaign team, to former House Speaker Nancy Pelosi, Senate majority leader Chuck Schumer and House Minority Leader Hakeem Jeffries," said one longtime Biden bundler.

"While Biden doesn't really care what donors think, the leadership does, because of the impact down ballot," the bundler added.

Pelosi, Schumer and Jeffries have each reportedly warned Biden about their members' concerns and said that his long odds in November will likely drag down congressional candidates if he stays in the race.

The Biden campaign did not reply to a request for comment. Representatives for Hillary and Bill Clinton and Zeke Emanuel, likewise, did not reply to requests for comment from CNBC. A press representative for Ari Emanuel declined to comment. Jones did not return emails seeking comment. 

Yet Biden's fundraising collapse does not necessarily spell doom for the party.

In an unexpected twist, events that feature Vice President Kamala Harris, Biden's likely successor should he step aside, have started to sell out.

An online seating chart for a concert event with Harris in Pittsfield, Mass. on July 27 shows it is almost entirely sold out. Tickets start at $100 and go up to just over $12,000, according to the invitation. Folk legend James Taylor and cello star Yo-Yo Ma are the headliners.