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: Includes only the Democratic Caucus. There was no Republican Caucus because President George W. Bush was an incumbent and the GOP nominee.