2000, 2008, 2016: Combined participation in the Democratic and Republican caucuses.
1996, 2012: Includes only the Republican caucus. In these years, there was/is no Democratic Caucus, because there was an incumbent president from the Democratic Party that took the nomination.
2004, 2020: Includes only the Democratic caucus. In 2004 there was no Republican Caucus because President George W. Bush was an incumbent and the GOP nominee. In 2020 there was technically a Republican caucus but it was largely not competitive and no entrance poll was conducted.