"The people of Alabama will do the right thing," Mr Trump tweeted. "Doug Jones is Pro-Abortion, weak on Crime, Military and Illegal Immigration, Bad for Gun Owners and Veterans and against the WALL."
He added: "Roy Moore will always vote with us. VOTE ROY MOORE!"
Alabama voters go to the polls on Tuesday to select a US Senator from their state in a special election. The race has been a contentious one, in which child sex abuse allegations have put Mr Moore, a Republican, at a disadvantage in the traditionally red state. Mr Moore denies the allegations.
Mr Moore's campaign has regularly emphasised his Democratic opponent's support for abortion rights – something a majority of Alabamians oppose.
Mr Jones' campaign page clearly states that he will "defend a woman’s right to choose and stand with Planned Parenthood". He has also defended allowing abortions after 20 weeks in the case of medical emergencies.
Mr Moore – who has proposed outlawing abortion and defunding Planned Parenthood – told Breitbart News that his opponent "goes to the extreme" on abortion rights.
"He is ultra-liberal on abortion. He is ultra-liberal on immigration. He is ultra-liberal on many things," Mr Moore said. "And we’ve got to either bring back conservativism or we will lose it all together.”
Still, some anti-abortion advocates say they cannot support Mr Moore because of the sex abuse allegations against him. Bama Students for Life, the antiabortion group at the University of Alabama, told the Washington Post that the group would not be backing the candidate.
"His actions are not pro-life and they are not acceptable," the group's Vice President told the Post. "We do not support him in any way."
Multiple women have accused Mr Moore of perusing them when they were teenagers and he was in his 30s. One woman claims he initiated sexual contact with her when she was only 14 – under the age of consent in Alabama.
Mr Moore has dismissed the allegations as politically motivated and "disgusting".
Citing these denials, Mr Trump threw his support behind the Senate candidate last month. The President told reporters that Mr Moore "totally denies" the allegations, adding: "He says it didn't happen. You have to listen to him also."
Shortly thereafter, Mr Trump tweeted his official endorsement of the candidate. He has since lobbied for Mr Moore at a rally in Florida, and recorded robocalls for the Moore campaign in Alabama.
"LAST thing the Make America Great Again Agenda needs is a Liberal Democrat in Senate where we have so little margin for victory already," Mr Trump tweeted last week. "...He’s bad on Crime, Life, Border, Vets, Guns & Military. VOTE ROY MOORE!"
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