I doubt a single SCOTUS vote will cause that huge a swing.
He might be looking for an excuse to reject him -- such as the conclusion that the ruling that doomed the Nixon Presidency was wrong. "I supported him until" is an effective way of showing bipartisanship in principle even if there must be exceptions.
Manchin I swear... you better vote for Kavanaugh! We aint losin ya buddy!
Sure, because it's totally worth having a Democrat in the Senate who doesn't vote with the Democrats when it counts.
Would you rather have someone who votes with your party 40% of the time, or someone who votes with your party 3% of the time? That's the difference between Manchin and Generic Republican. Morrisey is Generic Republican. Therefore, if I were a Democrat, I would be expressing my support for Manchin as well.