Trump had previously characterized the allegations, first reported by The Washington Post, as "very troubling" before changing tack and warning voters in Moore's home state of Alabama that a victory for his Democratic rival Doug Jones "would be a disaster!"
On Monday, he cited Democrats' opposition to his legislative agenda, including tax cuts, as reasons for his support.
"Democrats refusal to give even one vote for massive Tax Cuts is why we need Republican Roy Moore to win in Alabama," he tweeted.
"We need his vote on stopping crime, illegal immigration, Border Wall, Military, Pro Life, V.A., Judges 2nd Amendment and more. No to Jones, a Pelosi/Schumer Puppet!"
"Look at your 401-k's since Election. Highest Stock Market EVER! Jobs are roaring back!" he added, referring to an employer-sponsored retirement savings plan.
Moore, a 70-year-old Christian conservative with a history of controversy stemming from his tenure on Alabama's supreme court, had been a strong favorite to win the rightwing state's special election on December 12 before the allegations broke.
Most of the allegations about Moore relate to when he was a prosecutor in his mid-30s and sought to pursue relationships with teens, according to the reports.
One of the women, Leigh Corfman, now 53, said that when she was 14 Moore took her into his house in the woods near Gadsden, Alabama, removed her shirt and pants, and fondled her over her bra and underpants.
Moore has denied the allegations and said they are politically motivated.
The race, which is being held to replace Jeff Sessions who was named US attorney general, has national repercussions because Republicans hold only a slim 52-to-48 majority in the senate.
Allegations of sexual harassment have plagued both of America's main political parties in recent weeks.
Democrat John Conyers, a celebrated civil rights activist who is the longest-serving member of Congress, announced he was stepping down from a leadership position last month as he battles harassment claims from members of his staff.
Trump himself has been accused of misconduct by several women, ranging from groping to unwanted advances and kissing, though these did not prevent his election last year.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)