They only need 51 votes to make a rule change. Hence they’re putting massive pressure on Manchin and Sinema (who are both Democrats but notably understand both the long term view and are in swing states) to cave.
Basically, if they succeed, it sets off a raft of leftist legislation to change voting rules (because they’re the Democrats’ only hope of retaining power in the wake of what will be massively unpopular laws), and then, assuming the GOP gets a trifecta, they’ll be able to make all sorts of changes the Democrats won’t be able to block.
@Cherico is right that the Democrats can’t think long term besides “suppressing opposition” which is why, should they do this, it’ll only make matters worse, especially when (not if) it blows up in their faces (assuming they repeal it).
So, we shall see. Manchin is one vote, but so is Sinema and either one can throw a wrench into this. Not guaranteed it’ll happen, but who knows.