Why the Lead Changes: Simulating Blue and Red Shifts in Vote Counting
R. Michael Alvarez · California Institute of Technology
In many states, a large share of ballots is counted after election night: mail ballots that arrive or are dropped off late, and provisional ballots that must be checked first. The voters who cast those ballots tend to differ from early voters, so the lead can change as counting continues without anything having gone wrong. This simulation lets you change who votes which way and the order ballots are counted, and watch the lead move.
Simulation only: invented ballots, not real election results or a forecast.
How to use this page
Explore
Pick a scenario button to load a ready-made example.
Move the sliders: how younger and older voters lean, and how big and how young each batch of ballots is.
Change the counting order and compare the election-night lead with the final result.
Call the race
Pick a round. Everyone who picks the same round sees the same race, so a class can play together.
Press Next report to release results one batch at a time.
Call the winner when you're sure. A correct call scores 100 minus the percent counted; a wrong call scores zero.
How the simulation works
500,000 ballots fall into four batches: early mail, in person, late mail, and provisional.
Each batch's Democratic share is set by its mix of voters under and over 45, plus an optional “party strategy” effect for years when parties pushed different voting methods.
Batches are reported in the order you choose. Nothing about the voters changes between counting orders.
In Call the race, each report adds a little random variation, as real precincts do.
Scenario
How each age group votes (Democratic share of the two-party vote)
End of election night
Final count
Shift after election night
Toward the party named
Simulated data. These are invented ballots for teaching, not results or a forecast for any real election.
Call the race
Reports come in one at a time, just as on election night. Call the winner when you're confident: a correct call scores more the earlier you make it, and a wrong call scores zero. Everyone who picks the same round sees the same race.
Your score this round
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Points = 100 − percent counted when you called (wrong call = 0).
Counted so far
0%
Current margin
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Latest report
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Reports released so far
Report
Ballot type
Counted
Margin
Simulated data. Each round is an invented race, not a real election.
Glossary
Blue shift / red shift
Movement in the vote margin toward the Democrat (blue) or the Republican (red) after election night, as more ballots are counted.
Vote-by-mail ballot
A ballot mailed to the voter and returned by mail, at a drop box, or at a vote center. Ballots that arrive early can be processed before Election Day; ones that arrive late are counted afterward.
Provisional ballot
A ballot cast in person when a voter's eligibility can't be confirmed on the spot. It is counted only after election officials verify it, often days or weeks later.
Election night
The first results released after polls close. They usually include early mail ballots and in-person votes, but not everything.
Canvass
The official process after Election Day of processing and counting the remaining ballots and checking the totals.
Certification
The formal approval of the final, official results once the canvass is complete.