It’s actually not a one-party state, it’s a “Dominant Party System” which is defined as a system where there is “a category of parties/political organizations that have successively won election victories and whose future defeat cannot be envisaged or is unlikely for the foreseeable future.”
Examples in the Western world include the Swedish Social Democratic Party in Sweden from 1932 to 1976, and the Norwegian Labour Party ruling from 1935 to 1965
Dominant Party is a good way to put it.
In the meantime, the GOP will be dragged inexorably Leftwards in order to remain viable.