Any incumbent below 50% is in trouble.
Not when they are at 49%, though. Undecideds usually break for the challenger heavily, but they could break 10-1 for the Republican in this case and Manchin still wins if he is truly at 49% now.
Ads need to be run pointing out that while many like Joe, he is a vote for Chuck Schumer and if Chuck Schumer becomes majority leader, it doesn’t matter where Joe claims to stand, it won’t be allowed to even be brought to a vote.