This really sucks for McCain because the endorsement will be on the news for the next few days, which McCain desperately needed to change the narrative of "McCain is going to lose". Even if the endorsement itself doesn't sway anyone, it takes free time away from McCain that he can use to change the media narrative, and every second he can get is precious when the election is almost two weeks away.
That said, unless there's some serious election fraud or a huge Obama-related scandal, McCain is in deep doo-doo.