You expect him to endorse a Democrat?
He should do what Bush 41 did to David Duke. Some candidates are so disgusting that I would hope not even the President from the same party would support them (of course, Trump himself fits with such candidates imo).
The difference between Duke and Moore is that Duke was a known quantity. His Klan activities and White Supremacist ideology was well known and documented.
While I personally believe the Moore accuser that was 14 at the time, I can understand why people don't. It IS a 40 year old accusation. Moore HAS been a front-line public figure in Alabama for decades. While it normally sounds juvenile to hear a "They just want to get me in trouble!" defense from folks accused of serious things, there is a certain logic to that when a political figure is accused, as opposed to an ordinary person. There is something in it for a lot of folks to get Moore in trouble.
What I wish Trump would be is more temperate on other people who are accused. I wish he would have been more temperate with Bowe Bergdahl, who whatever wrong he may have actually done, suffered hell on earth for years while in captivity. I wish he would have been more temperate with "The Kneelers" recognizing the legitimacy of the issue (for some) behind a wrongheaded action, and not call them sons of bitches. And I wish that he could have spared us the "soft on crime" rap on Doug Jones. Jones symbolizes making right the wrongs of Jim Crow Justice.