What to know on Election Day 2020: Culmination of the battle for the White House

Voters are casting their ballots nationwide Tuesday to choose whether the next president of the United States will be Donald Trump or Joe Biden, even as tens of millions have already voted either early or through the mail.

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Here's everything you need to know for Election Day 2020 as Americans choose their next president. 

Battleground states

Every election year has so-called "battleground states" and their importance comes to the forefront.

Because most states assign their Electoral College votes on a winner-take-all basis, this means states that are reliably red, like Oklahoma, or reliably blue, like California, are considered baked into each candidate's path to victory.

Therefore the candidates focus their time and effort on politically moderate states that could swing for one candidate or the other, and thus are likely to put them over the 270 electoral vote mark to secure the White House. 

This year perhaps the most important battleground state is Pennsylvania, which provides a meaty 20 electoral votes and could provide an early indication of whether other Rust Belt states will swing for Trump or for Biden. If Trump can notch a win there -- and by extension potentially Michigan or Wisconsin -- it would put him on a very strong path to a second term. 

Meanwhile, a win for Biden in Pennsylvania almost certainly forecloses a Trump path to victory.

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In fact, because more states are likely in Biden's camp, according to polls, Trump will likely need to run the table with a large number of battleground states.

These include Texas, which after a long time as a solidly Republican state has become more purple in recent years; Ohio, a longtime swing state that no Republican has ever won the presidency without; and Florida, which along with Ohio has long been one of the most important states in presidential elections. 

States that are more important to Biden's path to victory include Michigan and Wisconsin, as well as Minnesota, which Trump narrowly lost in 2016, and Nevada. 

Georgia, Arizona and North Carolina  -- as well as a handful of other moderate states --  also figure to potentially play an important role Tuesday night, depending on the Electoral College scenario.

Poll closing times

The first states to have all polls closed will come Tuesday at 7 p.m. ET in Georgia, Indiana, Kentucky, South Carolina, Vermont and Virginia. (Kentucky and Indiana will have some polls close in the Eastern Time parts of the state at 6 p.m.) The last polls of the night will close at 1 a.m. ET in Alaska. 

Georgia is the only battleground state that will see all its polls close at 7 p.m. E.T., but it will be followed by a flurry of other critical states. Florida's polls close at 7 p.m., local time, which means full-state results from the state will not begin to feed in until 8 p.m. E.T., as part of Florida is in the Central Time zone.

Polls in North Carolina and Ohio close at 7:30 p.m. E.T.

Maine, New Hampshire and Pennsylvania close their polls at 8 p.m. E.T. Texas closes its polls 7 p.m. local time, which means in most of the states polls will close at 8 p.m. E.T. Some of the far-western parts of Texas are in Mountain Time, and will see their polls close 9 p.m. E.T. All polls in Florida will be closed by 8 p.m. E.T. 

Arizona, Michigan, Nebraska, Minnesota, and Wisconsin all close polls at 9 p.m. E.T. All polls in Texas will be closed by this time. 

Iowa and Nevada close their polls at 10 p.m. E.T.

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To find details about other states' poll closings, click here. 

A wait for results?

Amid warnings that the time-consuming process of counting mail ballots could take days in some states -- and plans by some states to accept ballots that are sent by Election Day but received days later -- Trump has demanded "a final total on November 3rd."

But it's not clear that will be the case -- because of how states count their ballots and how media organizations decide to call the races. 

Most states in the past have been able to count the vast majority of their ballots on the night of the election after polls close. But in almost all states it takes days or even more for every ballot to be counted and the race results to be certified and official -- the final total. Indeed, no state certifies a final vote total by election night. 

But as states report their results, news organizations are competing to see which one can be first to make an unofficial "call" of an election. The organizations look at votes for each candidate as the incomplete totals are reported, the percentage of votes that have been reported, polls and how a state has voted in the past. 

When, based on that confluence of factors, those organizations have enough confidence to say that the trailing candidate cannot make a comeback as more votes are counted, they will "call" a race. 

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Those calls, no matter the outlet, do not represent the official result of the election. They represent the assessment of experts hired by those organizations that even though the state's election results are not yet official, one candidate is sure to win. 

But in 2020, many states, including key swing states, may count their votes more slowly than during typical years. There are also cases -- such as in Pennsylvania and North Carolina -- where elections boards will count ballots that are mailed on or before Election Day but are received days later. Pennsylvania will accept ballots for up to three days after Election Day and North Carolina will accept ballots for nine days. 

If races are close in those states or others that could take longer to count their votes, the late-counted ballots could represent enough votes to close a lead one candidate or another may have on Election Day. If that is the case, it is likely news organizations will not make race calls on Nov. 3 and wait until a higher percentage of ballots is counted. 

Delayed calls by news organizations will not represent fraud or anything unseemly, then, but reflect the fact that states are taking longer to count votes because it is more labor-intensive to count mailed ballots. 

Swing one way, then another

Traditionally Democrats have done more early and mail voting than Republicans, who largely prefer to vote in-person. That effect has been amplified by the coronavirus pandemic, according to reports, as use of those methods has increased by those wishing to stay away from the polls.

This means that there will be chunks of votes reported in each state that may disproportionately favor one candidate or the other, giving the illusion that either Biden or Trump is leading by more than they might otherwise be. 

States that report their early and mail-in votes immediately after polls close will likely start with a swing toward Biden, then see numbers get closer as the night goes on while in-person votes are counted. States that take longer to count their mail votes are likely to see vote counts that favor Trump early on Tuesday night as in-person totals come in and then a swing toward Biden later in the evening and potentially in the following days. 

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The possibility of these swings could be a further reason why race calls may be delayed late Tuesday night into Wednesday morning -- or even later than that. 

Senate races to watch

Meanwhile, 35 of the 100 U.S. Senate seats are up for election this year, and the map has Republicans, who currently hold a 53-45-2 majority in the Senate, on the defensive. There are 12 Democrat-held seats and 23 Republican-held seats up for election.

With incumbent Sen. Doug Jones, D-Ala., expected to lose to Republican challenger Tommy Tuberville after winning a special election in 2017 against the disgraced Roy Moore, Democrats will need to unseat five Republicans to gain control of the Senate if Trump wins -- and four if Biden wins. 

Senate Majority Leader Mitch McConnell, R-Ky., has said the race for control of the Senate is essentially a tossup with many of his incumbents in challenging races. 

Sen. Susan Collins, R-Maine, is facing the political fight of her life against state House Speaker Sara Gideon, a Democrat.

Both the Georgia Senate races are expected to be close. One is between Sen. David Perdue, R-Ga., and Democrat Jon Ossoff. There's also a special election between Republicans Rep. Doug Collins and Sen. Kelly Loeffler, and Democrat challenger Raphael Warnock. The race for the Loeffler seat is likely to be decided in a January runoff. 

Senate Judiciary Committee Chairman Lindsey Graham, R-S.C., will see if shepherding Justice Amy Coney Barrett through her confirmation process is enough to energize his base and fend off well-funded Democrat challenger Jaime Harrison. 

Sen. Thom Tillis, R-N.C., could be unseated in his race despite the fact that major scandals have nagged his opponent, Democrat Cal Cunningham. 

Democrat Mark Kelly is considered one of the most likely Democrats to flip a red seat blue in his fight against incumbent Sen. Martha McSally, R-Ariz. 

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Sen. Cory Gardner, R-Colo., is facing off with another high-profile politician in his state, former Democratic Gov. John Hickenlooper. 

Sen. Gary Peters, D-Mich., has been favored to win his race but is facing an increasingly competitive challenge from Republican John James in what is one of the top Republican pickup opportunities. 

Democrat Theresa Greenfield has run a well-funded challenge to Sen. Joni Ernst, R-Iowa, whose seat is considered one of the most important for Republicans to keep if they aim to hold on to their Senate majority. 

Texas Republican Sen. John Cornyn is favored to win his race against Democrat MJ Hegar, but could face a close race in a state that has become more purple in recent years. 

Sen. Steve Daines, R-Mont., is facing a similar challenge as Gardner from a former governor. In Daines' case that former governor is Democrat Steve Bullock. 

The results of these races, as well as others -- including in Alaska, Kansas and Minnesota -- will decide whether Republicans or Democrats end up controlling the U.S. Senate, which could serve as a bulwark against or a support for the agenda of whoever is president after Inauguration Day. 

The House of Representatives is widely expected to remain in Democratic hands.