Though I suppose it depends what you mean by "an extended period of time".
I'd say that a generation - twenty five years, or five election cycles - would be sufficient. Roosevelt never made great inroads in the region, and usually won by only a few points in most states; he never dominated there the way he did in most of the rest of the country.
I don't know - Montana and Utah were always pretty strong, even towards the end of what might be thought of as the New Deal electoral period. But as far as longer term domination goes, we're probably dealing with (possibly indirect) economic factors to an extent. More interesting, to me at least, would be the big falls in Democratic support in the interior West at all levels since the 1970's or so.
