Harkin’s strength comes from industrialized and Catholic eastern Iowa.
The Republican Party at its founding was not conservative. Neither was the Republican Party that was part of the Progressive Movement. The Iowa GOP has mostly been home to ‘moderates’, with a few exceptions like H.R. Gross and Steve King.
Iowa caucuses do NOT pick pledged delegates to the national convention. They are local organizational meetings. (How do parties in other states pick local party officials?) All the attention is for a casual, non-binding presidential preference poll.
There was a Senator Jack Miller who was conservative too, but he got booted out with Nixon’s big win in 1972.