If Murkowski makes it through the primary, she will win re-election easily, regardless of the Manchin endorsement. If she doesn't make it through the primary, then whatever Trump-selected loon replaces her will probably win easily as well, and be far worse for Democrats. So if Manchin can help her win the primary that's probably a net positive for Dems since Murkowski at least works with us on stuff sometimes.
This isn't how the elections in Alaska work anymore. It's a jungle primary and then a top four election with rcv.