What analogy best explains the narrowing down of presidential candidates for a general election?
The analogy I find most often used is "separate the wheat from the chaff"; for which the term winnow is used rather than the expression.
Literally, winnow (1a(1)) means "to remove (something, such as chaff) by a current of air".
Figuratively, in the political sense,, winnow (2c) means "NARROW, REDUCE", as in "winnowed the field to four contenders".
A Google search for "winnow" "democratic" shows "About 147,000 results". At least, in the first few search pages, the term "winnow" appears in the title, in the sense of narrowing the field of candidates by various means.
What I don't really understand, though, is how they narrow this group of 20 people down to two by the time the general election rolls around. I've heard about caucuses and primary elections, but I still don't know how this helps in narrowing down the list of candidates. Who gets to nominate them and actually take part in them?
The initial "winnowing" (or narrowing) of the field is done through general lack of support for the individual candidates' campaigns. This lack of support is revealed through polling and campaign contributions. The lack of support may result from poor debate performance or "messaging"; that is, fewer people like what the candidate has to say.
During the "primaries" (See: United States presidential primary), the field is narrowed further by voters in the respective parties. It is during the "primaries" (or state caucuses) that delegates are selected to attend the nominating conventions for the respective political parties. Some state delegates are "pledged" to a particular candidate.
"Each party determines how many delegates it allocates to each state. Along with those "pledged" delegates chosen during the primaries and caucuses, state delegations to both the Democratic and Republican conventions also include "unpledged" delegates who have a vote."
Finally, at the United States presidential nominating convention, the field is narrowed to the two candidates representing the Democratic and Republican parties in the general election.
Generally, use of "presidential campaign nominating convention" refers to the two major parties' quadrennial events: the Democratic and Republican National Conventions. Some minor parties also select their nominees by convention, including the Green Party, the Socialist Party USA, the Libertarian Party, the Constitution Party, and the Reform Party USA.