A fifth woman has come forward, and says Moore attacked her when she was 16.
Previously, two women accused him of groping them when they were just 14.
Thune spoke with KELOLAND News about the accusations.
"These accusations that are being made seem to me to be very well-founded. His denials are less convincing, and the best thing he could do for the people of his state of Alabama and people of this country is to step aside," Thune said.
House Speaker Paul Ryan is also calling on Moore to quit, and says the allegations against him "are credible."
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Sen. John Thune (R-SD) is yet another prominent GOP leader to ask Alabama's U.S. Senate candidate Roy Moore to step aside, as more women accuse him of sexual assault.