TMZ has learned one top campaign official is privately telling at least one mega-donor ... "It's only a matter of time" before Biden bows out.

We're told the official is saying the focus has now shifted from singular support for a Biden re-election to, "Democrats have to retain The White House." In other words, it's no longer all about Biden.

The official says Biden still needs time to process the panic within his party, and some of that involves understanding polling trends. But, the official concedes, "There's little hope the polls are going to improve."

We're also told no one in the campaign has, up to last week, "considered a Plan B," but now everyone's scrambling.

And, we're told, the official thinks the likely person to step in if Biden withdraws is Kamala Harris, and part of it is "a money thing." The money Biden raised is earmarked for the Biden/Harris campaign, and there's a serious question if that money would be available to another candidate.