Yes it's unfair and D's always play dirty. They came out with the allegations 1 month before the election which was too late to clear his name.
Always remember, D's only play on smear campaigns and not on policy merits. It's been their playbook for decades and R's never learned how to counter it until Trump came along. They also reverted back to giving into the smears this AL election and of course lost.
Repubs need to learn to play fire with fire if they want to win against Ds.