(CNN) The 2020 election taught us an important lesson: the first results you see after the polls close on Election Night can be very different than the final outcome once all the votes are tallied, a process that can stretch on for days.
In some states, Republicans jumped out to an early lead in 2020, only to get swamped as Democratic-leaning mail ballots were counted, a phenomenon dubbed the "blue shift" or "red mirage." But in other states, Republicans gained as the vote-count filled in over several days, buoyed by redder batches of votes.
These shifts are bound to happen again Tuesday, as results trickle in from key midterm races.
This happens because of varying state-by-state rules for how people can vote, how and when election officials tally the votes, and how often new results are posted. It's also exacerbated by recent trends in voter preferences, with Democrats much more likely to vote-by-mail or use ballot dropboxes, while Republicans disproportionally favor going to the polls in-person on Election Day.
These quirks of the US election system are normal and somewhat predictable. And they aren't indicative of fraud or wrongdoing, despite what many prominent Republicans have falsely claimed.
When more people vote-by-mail or vote early, these "shifts" or "mirages" can become even more exaggerated. And this year has already seen tens of millions of ballots cast before Election Day.
Here's a breakdown of the shifts we might see in six battleground states -- featuring critical races that will decide Senate control. President Joe Biden flipped five of these six states in 2020, and they all saw prolonged vote-counts. But a note of caution: Nothing is set in stone. These aren't predictions, they're guideposts.
Arizona is one of the most important states this cycle. There are competitive races for Senate, governor and downballot statewide offices, including for attorney general and secretary of state. In all four contests, the Republican nominee has promoted the debunked conspiracy that the 2020 election was illegitimate.
A blue-to-red shift could play out in Arizona, in part because election officials can start processing mail ballots as soon as they are received. This is an arduous process -- election workers spend days opening envelopes, verifying signatures, and piling stacks of ballots for easy tabulation. (They can do this prep ahead of time, and start counting early, but the results aren't released until Election Day.)
There was a clear "red shift" in Arizona in 2020. Former President Donald Trump gained ground as more batches of votes were tallied after Election Night, but Trump never overtook Biden, who carried the state by roughly 10,000 ballots, or just about 0.3% of the vote.
The Grand Canyon State has a long and bipartisan history of mail-in voting. But that trend started changing in 2020, with some Republicans eschewing the method because of Trump's false claims of fraud. Now, mail-in voting is more popular among Democrats, making the early results look "bluer."
The first reported results on Election Night generally reflect the earliest mail ballots that were cast, disproportionally coming from Democrats who are more enthusiastic about postal voting. Later waves of results will come from in-person polling places or mail ballots that arrived on Election Day -- which will likely skew Republican, as they did in 2020.
But these "shifts" can be unpredictable. In 2018, the later waves of results helped Democrat Kyrsten Sinema beat incumbent GOP Sen. Martha McSally. The race was called six days after Election Day.
A "red mirage' is expected in Pennsylvania, where there are marquee races for Senate and governor.
This played out in dramatic fashion in 2020. Trump was ahead by nearly 700,000 votes on Election Night, but over four painstaking days, his lead evaporated as mail-in votes were tallied in the major population centers of Philadelphia and Pittsburgh. Biden won the Keystone State by about 81,000 votes, and his projected victory there, on the Saturday after Election Day, was when he clinched the White House.
There was such an extreme shift because, among other reasons, election workers in Pennsylvania can't start processing mail-in ballots until Election Day. State Democrats have tried for years to relax these rules, which would lead to faster results on Election Night. The GOP-controlled legislature passed a bill last year making this tweak, but it was vetoed by Pennsylvania's Democratic governor because it also scaled back mail-in voting and imposed voter ID requirements.
However, there is a new state policy that incentivizes so-called "marathon counting."
The state offered grants to counties that pledge to continuously count votes after the polls close -- instead of going home in the middle of the night and restarting Wednesday morning. All but four of the commonwealth's 67 counties took advantage of this deal. The new policy should speed up the count.
Two years ago, Georgia helped flip the US Senate, and the White House, into Democratic hands.
Biden and the two Democratic nominees for Senate all prevailed in very close races. (Biden's victory came in November, while the Senate candidates won runoffs in January 2021.) They all padded their numbers as more ballots were counted over time, overcoming the "red mirage" from the early results.
A similar "blue shift" is expected this year, where there are close races for Senate and for governor, featuring incumbent GOP Gov. Brian Kemp in a rematch against Democratic nominee Stacey Abrams.
But the shift might not be as sharp this time around. The share of Georgians voting by mail will likely be smaller in 2022 than in 2020, because the Covid-19 pandemic has subsided. And election officials now have more experience with tabulating mail-in votes, leading to a faster count. These and other factors will likely blunt the impact of the "blue shift."
As the results start pouring in on Tuesday night, remember that Georgia is a runoff state. If no candidate passes 50% in a specific race, then the top two finishers will compete in a runoff election next month.
The situation is unclear in Nevada, where there are competitive races for Senate and governor. Like Arizona in 2020, there was a noticeable blue-to-red shift in Nevada years ago -- with Trump narrowing the lead over time, but not netting enough votes to overtake Biden.
It's unclear if this post-election dynamic will repeat this year.
Election officials in the state have not released many details about the vote-counting process, like which types of ballots will be reported first, and which will be reported later. This information is essential to figuring out the possible complexion of the early vote, compared with the later-reporting figures.
Furthermore, this is the first midterm election in Nevada with universal mail-in voting. The state adopted this system in 2020 when the Covid-19 pandemic hit, and it's still in place for the 2022 cycle.
Another variable are the ballots postmarked by Election Day that arrive at election offices after the polls close. Some people call these "late-arriving ballots." In Nevada, they're still completely legal votes and get counted, as long as they arrive by November 12.
All of this uncertainty -- from figuring out which kinds of ballots have been counted already, and how many legal votes will arrive after Election Day -- will make it challenging to project winners on Tuesday night.
Wisconsin is another state with a likely "blue shift," with votes looking better for Democrats over time.
But it's a relatively small state, and its vote-count tends to go quickly. Wisconsin relies on thousands of local clerks to administer the election, and they usually wrap up most of the counting on election night.
There was a "blue shift" in Biden's favor in 2020, and his victory became clear within 24 hours. Of the five states Biden flipped in 2020, Wisconsin was the "first," in the sense that it was the first Trump-to-Biden flip that the news networks were able to project..
We'll see how the post-election shifts shape up in the Badger State, where there is a competitive Senate race featuring a Republican incumbent, and a close governor's race with a Democratic incumbent.
In Michigan, there might be a red-to-blue shift.
State lawmakers passed a law last month letting many localities process their mail-in ballots before Election Day. While many clerks aren't taking advantage of the extra processing time, some of the largest cities in the state are — including Detroit and Grand Rapids, the two biggest cities.
This should speed up the count. It means we'll likely see a fuller picture of the overall results on Election Night.
Two years ago, Trump was leading in Michigan when voters went to bed, but Biden pulled ahead the next day, and the networks soon projected him as the winner. The Michigan secretary of state's office said Monday that it could again take up to 24 hours for this year's full results to be reported, though the smaller counties might wrap up earlier than that.
Biden was helped by late waves of ballots from Detroit, a Democratic stronghold that historically counts its votes slower than other jurisdictions, because it's the largest city. Results from Detroit will be critical in determining if incumbent Gov. Gretchen Whitmer, a Democrat, fends off GOP nominee Tudor Dixon.
CORRECTION: An earlier version of this story incorrectly stated the margin of votes by which President Joe Biden won Pennsylvania in 2020.