I don't think Tanden's going to really struggle to get enough votes anyway. Manchin's just being silly and is going to look like a derp when even some Republicans vote to confirm her.
I think it’s certainly possible she doesn’t get confirmed. Are we sure Manchin is the only D-no at this point? Not everyone has publicly taken a position, and there are a couple Senators on the left or right edges of the party that I could see voting no (especially now that they wouldn’t be the first one to announce it). On the other hand, there are 2 or 3 Republicans who might want to boost their “bipartisanship” by putting the nominee over the top.