NYT is reporting on a hastily arranged call of the DNC finance committee today with some big donors. I don't see a non-paywalled source about it, but I assume it is happening.
That is a hugely important meeting because if Biden's donors say they will not support the campaign financially anymore, that will be one of the dominoes that would need to fall to get Biden to back out. Campaigns are expensive, and Trump's fundraising is focused on his campaign as third priority, after paying his legal bills and Trump himself.
I'm not really sure whether Biden can pull this off or not, but his chances are zero if his big donors stop giving.