Here’s the deal with the calling races on the night of the election: all of the networks have a >95% idea as to who’s going to win in each state (Nate Silver isn’t the only one who uses algorithms…). The networks use the National Election Pool (NEP) for exit polls and for projections. It’s been a while since I worked in this (my internship for CBS during college was awesome), but I’ll bet it hasn’t changed much:

• Each county (or similar jurisdiction – parish if Louisiana, borough in Alaska) has voting precincts. Some precincts have had the same boundaries for a number of years, even decades; others will be brand new this year because of the redistricting that happens after every census.
• The NEP picks precincts from the 31 states where exit polls are being held that are very “purple” in nature. These precincts have split their results between the two major parties for a quite a few elections.
• The NEP works with local election officials to get the results from these precincts to the NEP as quickly as possible on election night, usually straight from the polling place.
• In states that have polling places (all but OR and WA), they’ll have exit polls already tabulated (about three hours before the first results are announced) to verify the hard data they’ll get at the poll closing but also for questions like why people voted the way they did (giving the talking head class something to bloviate about).
• They do all of the number crunching and wait for results from the polling places which either verifies the exit polls (resulting in calling the election right away) or doesn’t (resulting in not calling the race for a while).

The majority of states will be called immediately after their polls close, but we’ll discuss tomorrow when they’re called by looking at the battleground states.

10.25.16 – updated in 2016 with the new URL for fivethirtyeight, removed the link for UnskewedPolls.com and kept the rest.