As FR pointed out, absolutely no one in Alaska is going to base their vote off of what Joe Manchin thinks, which means this is yet another attempt to get attention and showboat for his own personal gain. If he had said “Look, I respect Murkowski, I think she’s a great Senator, I don’t think it’s fair that she’s going to be primaried for voting in accordance with her convictions, she’s a good person” and stopped short of endorsing her, that would be, in my book, annoying but fine. A member of the Democratic Party—a Senator in a tied Senate nonetheless—advocating that a Republican be re-elected, no matter who it is or where they’re from, is flat out unacceptable. And I know he endorsed Collins last year too. Democrats should not be advocating to bring back Republicans—we should be advocating to bring in more Democrats. And we ought to be doing that by advocating against Republicans.
It’s not that I think this is going to seal the deal for Murkowski; that would be silly. It’s the principle that Manchin continues to do this knowing there’s no mechanism in place to stop him because he’s the 50th vote. And a lot of people on here will argue that saying “he’s not a Democrat” is unfair, but I can’t even imagine if Joe Biden came out and literally endorsed a Republican over a Democrat in a Congressional race. It would never happen. Manchin is the only Democrat that does this as often as he does. I understand he’s better than the alternatives in WV, but I’m eager to strengthen the Senate majority so that what he thinks doesn’t matter anymore.
Joe Biden endorsed Fred Upton in 2018 IIRC 
He didn't officially endorse him, he just praised him once at a speaking event. Obviously still not good given how competitive Upton's race was at the time, of course.
