Thanks to GOP pollster David Winston, who has gathered national and state exit polling data from 1984-2012 right here, we can do that in a single chart. Here's Winston's chart of national party ID over the past three decades as culled from exit polls:
What's remarkable is the consistency of the percentage of voters calling themselves Democrats over that time period. In those eight presidential elections, Democratic party ID has never dipped below 37 percent and never risen above 39 percent.
There has been more fluctuation in Republican party ID over that time. Republicans reached 37 percent of the electorate in the 2004 election but have dipped to 32 percent in each of the last two presidential contests -- the party's lowest ebb in 30 years.
The number of people identifying as independents has increased very slightly; 29 percent of the electorate said they were independents in 2012, equaling 2008 for the largest number in decades.
One thing worth noting: the gap between Democratic and Republican self identifiers has changed a bit over time. Between 1984 and 2004, Democrats never enjoyed a party ID edge larger than four points over Republicans. In 2008 that gap was seven points; in 2012 it was six points.
What lessons should we take from these numbers?
1) Democrats are likely to make up between 37 percent and 39 percent of the electorate in 2016 and almost certainly will outnumber the number of self identified Republicans. That doesn't guarantee victory, however. Republicans have won four of the last eight presidential elections despite the party ID differential.
2) Republicans, at least in the last two elections, have found themselves at a somewhat atypical low ebb in party ID, perhaps the result of the emerging split between their tea party and establishment wings.
3) For all the talk of the desire for a third party, there's been little major growth among those identifying as independents over the last three decades. (One could, of course, argue that if independents had a chance to vote for a third party candidate at the ballot box, the exit poll numbers might look differently.)