Very few politicians win their party’s nomination, lose the general election, turn around and win their party’s nomination again, and then win the general election their second time around. Nixon was the rare exception and he waited out an election cycle before running again. (Plus things did not turn out too well for Nixon after he finally won.)
OTOH, many successful politicians have lost their first race for their party’s nomination, watched the candidate who beat them go down in flames in the general election, and then ran for the nomination again, won the nomination, and then won the general election. They win their second run for the nomination in part because voters regret not nominating them the first time around.
Moore convinced Alabama Republicans that he was the best candidate for the general election. He then got trounced in the general election, when nearly any other possible Republican nominee would have won.
Yes Moore was the victim of scurrilous, unfair and defamatory attacks. Those attacks will only get louder if he runs again and he will end up pulling down other Republican candidates on the same ballot.
They cannot get louder. In 2017, the entire media-weapon was focused on Moore. In 2020, they will not be able to devote so much time to nasty hate-filled screeching at the man, because they will be spending that time directing nasty hate filled screeching at Trump and other GOP candidates.
The Democrat ceiling in Alabama appears to be around 700,000 votes, with Trump pulling in over 1.3 million. Whomever gets the nomination is likely going to get the vast majority of that 1.3 million Trump got in 2016, and so regardless of who it is, they will probably win.
Also, people saw them play this "sexual accusations" trick on Kavanaugh, and now people may very will be a lot less willing to believe these sort of "out of the dark" smear accusations against the man.