Since the current primary-dominated era of presidential nominations began in 1972, more primary votes have been cast in the Democratic than the Republican contests in all but two elections, 1996 and 2000. Part of this is due to the lingering Democratic advantage in party registration in much of the country, as well as the Democrats' penchant for high gusto nominating contests.
http://www.centerforpolitics.org/crystalball/article.php?id=FRC2008011701