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.