I'm just gonna start throwing numbers out here for the 2016 election:
Census numbers as of 2014 (click on the XLS file links to open spreadsheets) tells us there's about 142 million registered voters. Out of a citizenry over 18 years of age - minimum age to vote - of 219 million, that about 64 percent of the population able to vote.
It is more difficult to determine actual voter registration by party because only 28 states (and DC) allow people to register a party preference. If we go by national polling self-identification - for Gallup in July 2015 it was 46 Democrat 41 Republican - and put that as a percentage to 142 million, you get 65 million Dems and 58 million GOP.
But that's not including NPA/Third party, so we need to include that... gotta find the chart for that ah here it is... this is weird in that it says 43 percent identify as Independent while 30 percent are Democrat and 26 percent are Republican. But that normally doesn't mesh with real-world numbers: actual voter registration in Florida for example has the party identification about 38 D- 34 R - 27 N. It's possible that due to the states that DO NOT require voters to identify to a party, most registered voters would think themselves independent, but that may not reflect on their direct enrollment to either major party at the state level.
If we take the Gallup poll that includes Indy voters at face value: that would be 61 million Indy/Third party voters, 42 million Democratic voters, 37 million Republican voters. That doesn't entirely reflect actual voter turnout...
If we go by the 2012 voter turnout demographics for the Presidential election by Party, it looks like this (via that Wiki article):
Make it a given (axiom) that the party members will vote heavily for their own party (which we see in that table) regardless of the year/candidate, it comes down to how the Independents/NPAs are going to vote. If the Democrats can keep (or even improve on) their 45-50 percentage to Republicans - close split of the non-aligned voters - they can pretty much keep their overall advantage of 38-to-32 percent total voters.
The trick then for both major parties in 2016 is to not only appeal to their base - as always - but to get enough of the Independents to swing to their side. The Republicans did get 50 percent of the Indy vote but it wasn't enough to win enough states or the popular vote overall. The Democrats just have to keep the 45 percent of Indy votes they appeased last cycle while keeping their own base appeased as well.
By that logic, the Democrats have the easier path. The Republicans have to do a better job of appealing to NPA/Indy voters... which is next-to-impossible if their party platform is an extremist one. By an October 2015 WSJ/NBC News poll, self-identified Independent voters viewed Republicans as "out-of-mainstream" on five (finance, abortion, gay marriage, immigration, climate change) of six issues (the only mainstream view being about guns). If that view remains fixed by October 2016, there's little to shift Indy voters more to the GOP to change those 2012 numbers.
If we go by map it looked like this:
|from the 270towin.com site|
This is why the Democrats are focusing on repeating 2012 turnout as best as possible: it not only guarantees the popular vote win but also the Electoral College win. As long as the states that voted for Obama still vote for Hillary/Bernie, the Democrats keep control of the White House. Link to an interactive map to make your own prediction, by the by.
If there are any numbers that should bother both major parties, it's this: that same October 2015 poll I listed earlier also charted the Unfavorable numbers for the major candidates in both primaries.
|from Wall Street Journal/NBC News Oct. 2015 polling|
So what we're going to see in the 2016 election cycle is this: massive pandering by the Republican candidates to win their voting base, and then utter failure to swing back to the Moderate/Centrist facade because they caved too far Right Wing; utter silence by the Democratic Party as they try to keep a low profile, keep their far Left Wing happy with table scraps, and play to that Moderate/Centrist location to win over enough Indy voters to secure enough states for Electoral College wins.
Expect a lot of despair and frustration over the national ticket well into March when the Primaries actually kick in.
Thank GOD there's the NFL Rookie Draft in April to keep me distracted. Tampa Bay needs a lot of help at Defensive End and Cornerback this off-season, and Free Agency moves at DE have not panned out...