If Biden does this, will Manchin go Republican?
No.
I don’t think that would kick him over from the dark side, but I wouldn’t be the least surprised if Manchin does eventually switch parties.
Maybe after the midterms?
Manchin doesn’t seem to be in complete lockstep with the Left, and seems posture center/slightly center-right. Gut impression is that he’s not acting. Haven’t talked to family in WV in a real long time, but from what I gather it isn’t a lock for the Left anymore.