In the West, there used to be liberal Democrat senators, like Frank Church of Idaho and George McGovern of South Dakota. What allowed western liberals to do well in their states, and what prevented them from continuing to win elections?
As of 1997, the Democrats held one Senate seat each in Arkansas, Montana, Nebraska, New Mexico, Oregon, and Washington, two seats each in Nevada, North Dakota, South Dakota, California, and Hawaii. Of those 16 seats, 13 of them had been controlled by Democrats for at least the last two election cycles. One of the seats was won new from Republicans in 1992 (California), and two of them had been won new from Republicans in 1996 (Oregon, South Dakota).
So I think liberal Western Democrats still did exist in the American West, including in the American interior West, within the twenty year rule. To answer your question, you'd have to violate the twenty year rule of this sub.