Once the last voter in a state has voted and the polls close, the process of announcing the winner begins. That’s when Election Day turns into election night and each state begins voting. reports vote totals.

Some states, such as Florida, Georgia, and North Carolina, report their votes quickly, while other states, such as Arizona, Nevada, and California, take longer, often up to a week or two, for most ballots to be tabulated. In many states, patterns in how votes are reported can make it difficult to tell who the eventual winner will be in the middle of election night.

For example, vote returns may skew toward a party earlier in the night because only a particular party medicine First reporting of votes; for example, counting mail ballots before in-person Election Day voting. These patterns can sometimes create what are called “voting mirages.”

A. vote mirage is when the current vote count shows that a candidate received a higher percentage of the vote than he or she could have received ultimately resulting in the final count. Mirages can be “red” or “blue” depending on which side benefits – but what really matters here is that they “look”, because vote mirages, like other mirages, are temporary and do not reflect ultimate reality.

Factors creating the mirage of votes

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

Mirages may arise due to geographic differences in counties reporting faster or slower. It is common for small rural counties or precincts to report their counts more quickly than large urban counties because the logistics of counting ballots are easier when there are fewer ballots available. As a result, a red mirage can occur when a state’s vote count overrepresents rural areas at a certain point on election night, as voters in rural areas are more likely to be Republican than voters in urban areas.

Virginia is an example of a state that often experiences a red mirage based on geography early on election night. That’s because Fairfax County, a highly populated suburb of Washington, D.C., where Joe Biden won 70% of the vote in 2020, tends to be one of the last counties in the state to report the majority of results. Until Fairfax County reports most or all of its votes, Donald Trump will likely have a higher percentage of the vote than his final statewide total.

Fairfax County reported results for the majority of votes cast on election night 2020 (about 375,000 votes) at 11:43 PM ET. This single report caused Trump’s percentage of the statewide vote to drop from 50.2% to 45.8% at that moment. (This is close to Trump’s final vote share in Virginia of 44%.)

Another factor that can cause voting mirages is partisan differences based on voting pattern. voting by mail, early in-person voting, Election Day in-person votingand so on.

Many states and counties tend to report results from each mode one by one. If one party is more likely to cast their vote using a particular mode than the other party, this can lead to vote mirages. That 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 great blue mirage in North Carolina in 2020. In the first hour after the polls closed, most North Carolina counties reported mail-in and early in-person voting. Those votes were predominantly Democratic, with Biden at 57.6% of the statewide vote count at 8:18 p.m. ET. But over the next three hours, the state reported heavy Republican Election Day voting, causing Biden’s vote percentage to drop to 48.6%, the final figure here.

The third factor that can lead to voting mirages is more complex: Differences in the order in which specific ballots are counted. This tends to be the most important result States and counties reporting results of mail-in ballots in small batches.

Many election offices tend to count mail-in ballots in the order they were received; This means ballots returned by voters in mid-October will appear in vote counts earlier than those received on Election Day. If there are partisan differences between the first mail-in ballots and ballots sent later, this will lead to a voting mirage.

That’s what’s happening in Arizona in 2020. Mail ballots reported by Maricopa County (representing 60% of the state’s voters) on election night included those returned by voters the weekend before Election Day. This created a blue mirage because earlier mail-in votes were more Democratic than later ones.

The results of the mail-in ballots were then announced in the days following Election Day. As late-arriving mail votes were heavily pro-Republican, Donald Trump’s vote count rose from 46.8% to a final vote share of 48.9% at 3 a.m. ET on election night; This was slightly below Biden’s 49.2%.

Interestingly, Arizona’s trajectory in the 2018 midterm elections was exactly the opposite. Late-arriving mail votes were much more heavily pro-Democratic, and when they were added in, Democrat Kyrsten Sinema went from being down 0.8 percentage points on Election Night to winning the election by 2 points.

What to look for in terms of mirages in each state?

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

We’re also cautious about assuming historical patterns will hold true in 2024, especially given that the 2020 election was held during a pandemic, voters were more likely to vote by mail, and social distancing affected how election officials tabulated results. This year, Trump’s campaign encouraged his supporters to vote early, which could impact partisan differences between voting modes.

Here are specific details about the mirages we’ve seen in major swing states in the past and what could happen on election night this year.

North Carolina: Most mail-in votes will likely be reported first, resulting in a blue mirage. In a change to this year’s election, in-person early voting cannot be done until polls close on election day. In person, we expect early votes to start arriving around 8 p.m. ET, a half-hour after polls close in the state. However, as Election Day votes are reported in the evening, vote percentages will shift towards Republicans.

Georgia: Most counties will report in-person and mail ballots first, likely causing a blue mirage as Election Day votes are reported before statewide results tip toward Trump. After that, there could be a small swing back toward Democrats due to late-arriving mail ballots being reported from Atlanta’s major metro areas. Unlike 2020, Georgia counties are not allowed to pause the vote counting process until all ballots are counted.

Pennsylvania: We expect the first reported votes, including in heavily Democratic Philadelphia and Allegheny counties, to be dominated by mail ballots, causing an early blue mirage. Then, when in-person votes are reported on Election Day, the blue mirage will turn red and the results will point to more Republicans than we expected to see in the final margin. Finally, these larger counties (particularly Philadelphia) will continue to count and report remaining mail votes. This will likely shift the race in the Democrats’ direction.

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

Michigan: Expect a slight red mirage in Michigan as smaller, more pro-Republican localities report their votes. The vote total will likely increase as smaller 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 will be able to vote early in person for the first time in the general election, and clerks will now be allowed to pre-process mail ballots, making counting faster. Both of these changes could result in differences in vote reporting patterns compared to what we’ve seen in previous elections.

Wisconsin: In most Wisconsin municipalities, all forms of voting are reported together. This creates a slight red mirage because smaller rural municipalities tend to report faster.

A handful of large municipalities, including the city of Milwaukee, centrally tabulate mail-in ballots and report them separately from Election Day votes. The state will experience a red mirage until Democratic-dominated cities in the state report vote-by-mail votes with Democratic-dominated votes. Milwaukee reported at 4:42 a.m. on election night 2020 that Biden had received 85% of 168,000 mail ballots. This caused Biden’s statewide total to rise from 47.3% to 49.3%.

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

The blue mirage shrank to about 3.5 points as election night votes were announced over the next few hours. It continued to shrink after Election Day as pro-Trump mail ballots reportedly arrived late. In the 2022 Senate race, the Democrat was ahead by 19 points in the first hour and won by 5 points. But in the 2018 Senate race, the situation worked in the opposite direction; The Republican had a 3 point lead and was eventually defeated by 2 points. point.

In 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 reported its votes. In 2022, this difference was only 10 minutes. Then, the first report in Clark could create a blue mirage that will shrink as Election Day votes are tabulated statewide.

Late-arriving mail ballots will be notified in the days following Election Day. In 2020, these votes increased Biden’s lead from 0.6 percent to 2.4 points.

This story first appeared NBCNews.com. Read more from NBC News here: