President Biden is facing mounting calls from Democrats to exit the presidential race following his disastrous debate performance and potential replacements are being floated left and right. 

Biden has thus far rejected the pressure, insisting he could still beat Trump come November, but should the 81-year-old change course and bow out of the race, there are a series of hurdles other Democratic candidates could face.

One of the biggest challenges: how to raise a massive sum of cash to take on Trump in a short amount of time. Biden's massive campaign war chest would not necessarily flow to a replacement for the president at the top of the ticket.

And that is a considerable amount of money they could be leaving on the table.

Even as Biden has struggled in the polls and bombed the first debate, his campaign has been raking in cash.

The Biden campaign announced on Tuesday it raised $127 million last month, it's best fundraising month of the campaign to date. It had its best grassroots fundraising hour of the entire campaign from 11 pm to midnight, the night of the debate.

In the second quarter of the year, the Biden team brought in a total of $264 million and finished last month with $240 million cash on hand.

But if Biden exits the race, all that money does not just automatically go to the next Democratic presidential nominee.

Federal candidates can make unlimited transfers of their funds to the national party, so if Biden leaves the race, the money could all end up with the Democratic Party, which can then spend the money on the new nominee.

But the campaign cannot just transfer the money directly to the nominee, as there is a $2,000 limit per candidate per election. 

'In order for the withdrawing candidate’s committee to get the funds into the campaign account of a successor candidate’s campaign, the withdrawing candidate would have to refund the money to the original donors, who could then contribute to the new candidate’s campaign,' explained Trevor Potter, president of the Campaign Legal Center. 

Some major Democratic donors were holding off from making any decisions about the race immediately following Biden's performance as they waited to see what the debate would mean for the polling.

Others called for Biden to step aside following his abysmal debate.  

By Tuesday, some Democratic donors appeared to be circling the wagons, telling DailyMail.com they were going to get to work to elect Biden and the time for panic over the debate had passed.

Some of the names being floated as a potential replacements include Governors Gavin Newsom (California), Gretchen Whitmer (Michigan) and Josh Shapiro (Pennsylvania) as well as Transportation Secretary Pete Buttigieg.

But there is one name being floated who could use all that campaign cash as a replacement nominee without any hurdles: Vice President Kamala Harris. 

Campaign finance law allows her to share the campaign committee with Biden because the president and vice president run together as one ticket, so her name is already authorized.

'If President Biden were to cease being a candidate and was succeeded as the nominee by Harris, she would maintain access to all the funds in the committee and could use them to advance her presidential candidacy,' Potter said. 

The amount of cash at her disposal would be considerable as the campaign said it had $240 million at the end of June. 

More details on just how much of that is specifically in the Biden campaign committee versus the party will become more clear with the Federal Election filings later this month. 

On Tuesday, former Ohio Congressman Tim Ryan became the first prominent Democrat to publicly back Harris to replace Biden. He said she is ready for the job.

But the campaign cash laws is another potential point in her favor as well as new polling.

A CNN poll released post-debate looked at hypothetical matchups between Democrats and Trump should Biden not be the nominee. 

While all the Democrats trailed the ex-president, Harris came the closest in a head-to-head matchup: 47 percent to 45 percent among registered voters.

And now a Reuters poll has revealed that Michelle Obama could be the standout Democrat best poised to beat Trump - but there are even more hurdles to getting her the money.

If Harris were not the nominee, the the campaign committee could be converted into a political action committee or PAC. A PAC could make independent expenditures efforts could not be coordinated and a PAC can only make a $3,300 donation to another candidate per election. 

If Democrats were to replace Biden, campaign funds will be crucial for their fight to beat Trump.

The presumptive Republican presidential nominee ended June with nearly $285 million cash on hand according to his campaign, wiping out Biden's on-hand cash advantage with just four months to go before Election Day.

In total, the ex-president raised $331 million in the second quarter of the year including $111.8 million last month alone.