What's a ‘red' and ‘blue mirage,' and how election night vote counts make it hard to tell who will win

Once the last voter casts their ballot in a state and the polls close, the process of revealing the winner begins. That's when Election Day turns into election night, and each state starts reporting its vote totals.

Some states — like Florida, Georgia and North Carolina — report their vote quickly, while others like Arizona, Nevada and California typically take longer, upward of a week or two to tabulate most of their ballots. Within many states, the patterns of how votes are reported can make it difficult to tell in the middle of election night who the winner is going to be in the end.

For example, vote returns can skew toward one party early in a night because only a certain type of vote is reported first — like when mail ballots are counted before any in-person Election Day votes. These patterns can create what are sometimes referred to as "vote mirages."

A vote mirage is when the current vote count shows a candidate getting a higher percentage of the vote than they will ultimately end up with in the final count . Mirages can be "red" or "blue" depending on which party appears to be benefiting — but "appears" is the really important thing here, because like any other mirage, a vote mirage is fleeting and doesn't reflect the final reality.

The factors that produce a vote mirage

Generally speaking, there are three factors that produce vote mirages: geography, vote mode and order of tabulation.

Mirages can emerge because of geographic differences in the counties that report more quickly or more slowly. It's common for small, rural counties or precincts to report their tallies more quickly than large, urban ones, because the logistics of counting votes are simpler when there are fewer ballots to handle. As a result, since voters in rural areas are more likely to be Republican than voters in urban areas, a red mirage can happen when a state's vote count overrepresents rural areas at a certain point on election night.

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Virginia is an example of a state that usually has a geography-driven red mirage early on election night. This is because Fairfax County, a high-population suburb of Washington, D.C., where Joe Biden received 70% of the vote in 2020, tends to be one of the last counties in the state to report most of its results. Until Fairfax County has reported most or all of its votes, Donald Trump will likely have a statewide vote percentage that is higher than his final statewide total.

On election night 2020, Fairfax County reported the results from a majority of its ballots (nearly 375,000 votes) at 11:43 p.m. ET. This single report caused Trump's statewide vote percentage to shrink in that moment from 50.2% to 45.8%. (That's close to Trump's final vote share in Virginia, 44%.)

Another factor that can cause vote mirages is partisan differences by vote mode — mail ballots, early in-person votes, Election Day in-person votes , and so on.

Many states and counties tend to report the results of each mode one at a time. This can cause vote mirages if one party is more likely than the other to cast its votes using a particular mode. This was the case in 2020, when Democrats voted more by mail than Republicans, and Republicans were much more likely to show up on Election Day.

This difference caused a large blue mirage in North Carolina in 2020. In the first hour after polls closed, most North Carolina counties had reported their mail and early in-person votes. Those votes were heavily Democratic, and at 8:18 p.m. ET, Biden was at 57.6% in the statewide vote count. But over the next three hours, the state reported its heavily Republican Election Day votes, causing Biden's vote percentage to shrink to his final number there, 48.6%.

The third factor that can produce vote mirages is more complicated: differences in the order that specific modes of ballots are counted. This tends to be the most consequential in states and counties that report the results of their mail ballots in small batches .

Many election offices tend to count their mail ballots in the order that they were received, meaning that ballots returned by voters in mid-October will appear in the vote tallies earlier than those received on Election Day. This will produce a vote mirage if there are partisan differences between the earliest-returned mail ballots and the later-returned ones.

This is what occurred in Arizona in 2020. The mail ballots reported by Maricopa County (which accounts for 60% of the state's voters) on election night included those returned by voters by the weekend before Election Day. This ended up producing a blue mirage, because those earlier mail votes were more Democratic than the later ones.

The results of the later-returned mail ballots were reported in the days following Election Day. Because those late-arriving mail ballots were heavily pro-Republican, Donald Trump's vote count grew from 46.8% at 3 a.m. ET on election night to his ultimate vote share of 48.9% — just shy of Biden's 49.2%.

Interestingly, Arizona's pattern in the 2018 midterm election was the opposite. Late-arriving mail ballots were much more heavily pro-Democratic, and, once they were added, the Democrat, Kyrsten Sinema, went from trailing by 0.8 percentage points on Election Night to winning the election by 2 points.

What to watch for in terms of mirages in each state

The NBC News Decision Desk accounts for these mirages when deciding whether to project the winner in a race. For example, we are less likely to project a Republican winner in a state if all the remaining uncounted ballots are expected to come from heavily Democratic counties. Conversely, we are less likely to make a projection if the Democrat is ahead but most of the uncounted ballots are expected to be strongly Republican, in-person Election Day votes.

We are also cautious about assuming that historical patterns will hold in 2024, especially given that the 2020 election was held during a pandemic, with voters more likely to vote by mail and social distancing affecting how election officials tabulated the results. This year, the Trump campaign has also encouraged his supporters to vote early, which may affect partisan differences between vote modes.

Here are specific details about the mirages we have seen in key swing states in the past — and what could happen on election night this year.

North Carolina: Most of the mail ballots will likely be reported first, causing a blue mirage. In a change for this year's election, in-person early ballots cannot be tabulated until after polls close on Election Day. We expect the in-person early votes to start coming in around 8 p.m. ET, a half-hour after polls close in the state. But the vote percentages will shift toward Republicans as in-person Election Day votes get reported over the course of the evening.

Georgia: Most counties will report their early in-person and mail ballots first, likely causing a blue mirage before the statewide results move toward Trump as Election Day votes are reported. After that, there may be a small shift back toward the Democrats, as the late-arriving mail ballots are reported from large metro Atlanta counties. Unlike 2020, Georgia counties are not allowed to pause their vote-counting process until they have counted all ballots.

Pennsylvania: We expect the earliest-reported votes to be dominated by mail ballots, including from heavily Democratic Philadelphia and Allegheny counties, causing an early blue mirage. Then, as in-person Election Day votes are reported, the blue mirage will turn into a red one, with the results tipping more Republican than we expect to see in the final margin. Finally those large counties (particularly Philadelphia) will continue to count and report their remaining mail ballots. This will likely shift the race back in Democrats' direction.

In 2020, the Pennsylvania results at 3 a.m. ET on election night were about 7 percentage points more Republican than the final result. In the 2022 Senate election, the 3 a.m. red mirage was just over 1 percentage point from the final margin.

Michigan: We expect a slight red mirage in Michigan as smaller, more pro-Republican localities report their votes. The total vote count will likely trickle upward as small places report, and the size of the mirage will shrink when large, Democratic cities like Detroit, Grand Rapids and Ann Arbor report. This year, Michigan voters can vote early in person for the first time in a general election, and clerks are now permitted to preprocess their mail ballots, enabling faster counting. Both of these changes may cause differences in vote reporting patterns compared to what we have seen in previous elections.

Wisconsin: In most Wisconsin municipalities, all modes of vote are reported together. This produces a slight red mirage, because smaller, rural municipalities tend to report more quickly.

A handful of large municipalities, including the city of Milwaukee, tabulate their mail ballots centrally and report them separately from their Election Day votes. The state will experience a red mirage until the heavily Democratic mail ballots from most heavily Democratic cities in the state are reported. At 4:42 a.m. ET on election night 2020, Milwaukee reported that Biden had received 85% of its 168,000 mail ballots. This caused Biden's statewide total to increase from 47.3% to 49.3%.

Arizona: In 2020, about 70% of the state's votes — mostly early in-person and early-arriving mail ballots — were reported in the first hour after polls closed. This generated a statewide Biden lead of over 14 percentage points in a state he ultimately won by 0.3 points.

As Election Day votes were reported in the next few hours of election night, the size of the blue mirage shrunk to about 3.5 points. It continued to shrink after Election Day as the pro-Trump, late-arriving mail ballots were reported. In the 2022 Senate race, the Democrat was ahead in the first hour by 19 points and won by 5. But in the 2018 Senate race, the pattern worked in the opposite direction, with the Republican ahead by 3 points early before ultimately losing by 2 points.

Nevada: Nevada will experience a red mirage until Clark County (Las Vegas) reports. In 2020, Clark's first report came 90 minutes after the first county in the state had reported its votes. In 2022, this gap was only 10 minutes. Then, the first report in Clark may cause a blue mirage, which will shrink as in-person Election Day votes are tabulated statewide.

Late-arriving mail ballots will be reported in the days after Election Day. In 2020, these ballots moved Biden's lead from 0.6 percentage points to a final result of 2.4 points.

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