President Biden's campaign funding might slow to a trickle if he stays in the race ... at least according to a 6-figure donor who says democratic supporters are analyzing poll numbers and reacting.

Whitney Tilson's an author and investment expert who gives at least $100k a year to the Dems, but Thursday on "TMZ Live" he told us more and more donkey party donors are revolting -- over the president staying in the race -- the best way possible ... by closing their checkbooks.

Tilson explains donors aren't panicking because of the polls, but instead, they're making cold, calculated decisions based on concrete data -- numbers that show there's a low probability Biden's gonna win in November.

He says at least one top Democratic party official told him the internal polling -- which only Biden's campaign officials see -- is even worse than what the public sees.

As for support eroding around the prez ... Tilson says he thinks a vast majority of Democrats would say they want Biden to step down if their answer was kept under lock and key.

BTW, Whitney isn't dragging Biden, he says he's been a wonderful public servant, but no one he knows thinks 46 can do another full term.

Of course, George Clooney got many Democrats' attention with the op-ed he wrote asking Biden to drop out of the race ... and, other celebs like Michael Douglas are doubling down on George's request.

But, Whitney's intel is it's not just celeb support that's waning for Biden going forward.