In our system of "government" the real choice of who runs our country is made during the primary elections. This is because the propaganda mills we call "news media", state laws, and tribalism virtually guarantee that any given general election contest will essentially be a "choice" between a Republican or a Democrat--both of whom will have been chosen as the nominee during a primary election.

News outlets like to report on primary (and general) election turnout in the context of being relative to previous elections. They'll report things like "turnout is down from 2012" or "this state had the highest turnout of any (primary|general) election in its history!" What we rarely see reported is what the actual turnout numbers are. What they reveal is this: our government is deeply broken.

Primary and general election turnout data going back to 2000 is available from the United States Elections Project (for example; there are other sources). It paints an abysmal picture:

+-----------------+-----------------+------------+-----------------+
|   Election Year | Turnout Rate*   | Dem. Reg   | % Dem Primary   |
+=================+=================+============+=================+
|            2000 | 21.3%           | 33% + 11%  | 9.4%            |
+-----------------+-----------------+------------+-----------------+
|            2004 | 18.0%           | 34% + 13%  | 8.5%            |
+-----------------+-----------------+------------+-----------------+
|            2008 | 29.6%           | 34% + 17%  | 15.1%           |
+-----------------+-----------------+------------+-----------------+
|            2012 | 13.3%           | 30% + 19%  | 6.5%            |
+-----------------+-----------------+------------+-----------------+
|            2016 | 27.9%           | 31% + 17%  | 13.4%            |
+-----------------+-----------------+------------+-----------------+

(The registration percentage column data come from Pew Research and Gallup.)

The "Dem. Reg" column represents the percentage of voters registered as democrats (first number) and the percentage of voters registered as independent who lean democratic (second number).

The "% Dem Primary" column means "Percentage of eligible voters who voted in the democratic primary". It is taken by forming the product of "Turnout rate" and "Dem. Reg". That is, the number in this column represents the percentage of the eligible population who chose the Democratic nominee. Similar numbers apply to the Republican nominee.

Note: These are very rough numbers from the source. The "Turnout Rate" column is across all ballots counted, not by party, so there will be some minor discrepancies. Also, since some primaries are closed we really should only use registered democrats instead of adding the leaners in in those cases, so this table is actually being far more lenient than the data actually justify.

Let's let these numbers sink in. In any given election, fewer than 15% of the voting eligible population choose who the Democratic nominee for POTUS is. Due to variations on a state-by-state and district-by-district basis, these numbers aren't reflective of, e.g. Congressional primaries. It's actually much worse in many of those cases, because the numbers in the table are biased toward high-turnout states (like California). Some states, like Connecticut in 2004, see