James Udall, who also goes by the name Randy, was hiking in Wyoming's Wind River Range and was expected to come out Wednesday, the senator's office told the Washington publication Politico.
James Udall, a Carbondale, Colo., native, is an experienced trekker who has hiked alone in the mountains before.
Searches were conducted throughout the weekend.
"Mark is concerned about his brother's whereabouts and he continues to closely monitor the situation. He and his family hope for the best," the Colorado Democrat's office said.