Colorado tried a presidential primary for three nominating contests, did not consider it much of a success and dropped it before the 2004 election when paying for it became a state budget burden.
More Republicans participated in Colorado’s three primaries than Democrats, even with mild contests. But even with a primary only, about a third of the party voted. The first President Bush beat Pat Buchanan handily in 1992 and the 1996 contest was mostly over by the time of the Colorado primary, with Bob Dole beating Buchanan and Steve Forbes two-to-one. Same was true in 2000 when the second President Bush had already won the contest with John McCain.
Democrats in their big 1992 turnout gave a boost to Jerry Brown, with Bill Clinton and Paul Tsongas in close pursuit. The lack of a contest in 1996 (Clinton’s re-election) and the race being over in 2000 (Al Gore vs. Bill Bradley) produced lackluster participation.
Competition and timing are necessary to get voters engaged, especially Democrats. In 2008, the Democratic caucus had record turnout and was one of the important victories for Barack Obama. Republican caucus turnout was higher than usual in 2008 when Mitt Romney won against McCain and in 2012 when Rick Santorum led Romney and the field.
Denver Post: Colorado lawmakers want bigger role in 2016 presidential election
Ciruli Associates Variable – January 2000