The magic number is 270.
It really doesn't matter if you win the popular vote across the country. Just ask Al Gore, the defeated Democrat in 2000.
The US presidential election is decided on electoral college votes. Each state gets one electoral college vote for each congressional district it has and for each senate seat it holds.
READ MORE: How to read US polls
So New York with 27 members of Congress and two senators has 29 electoral college votes. North Carolina has 15. Washington DC gets the same number as the least populous state, so it has three.
Every state allocates their votes on a winner-takes-all basis, except Maine and Nebraska. There they use the "congressional district method" to select one electoral college winner within each congressional district by popular vote while giving two electoral college votes to the statewide winner.
That electoral maths give the Democrats a head start. In every election since 2000 they have won states that give them 242 electoral college votes. Republicans start with a virtually guaranteed 179.
Then the calculations begin. How to reach the magic number?
With Hillary Clinton ahead in polls, it will be hard for Donald Trump.
ANALYSIS: Understanding Trump's candidacy
Let's look at the polls and see what he needs to do.
Florida is the biggest prize of the night. Lose that and Trump supporters can prepare to go to bed.
However, he is ahead there. If he adds Ohio and Iowa, where he also has a narrow lead, and squeaks out Nevada where he's behind in early voting, then New Hampshire and North Carolina become critical.
He's behind in both places but hopes there is a silent majority in both which have been too embarrassed or too wary of declaring their support, and that they are many enough to carry the day.
If he does that, we would be in the incredible situation of a 269-269 tie.
Then Trump would be hoping to win the 2nd congressional district in the state of Maine which doesn't deliver its electoral college votes in a state wide, winner-takes-all block.
Given the polls, such a scenario is unlikely. But impossible?
Trump has spent time in Michigan, Wisconsin and even Virginia.
He believes his trade policies appeal to the white working classes in those states and that block has enough voters to peel away one of those reliably Democratic states.
If he succeeds, he changes the electoral college map and this is an entirely different game.
It is important to note that Mitt Romney thought he would be able to do the same four years ago and was much closer in some polls. He failed.
So Donald Trump has thrown himself into this final Election Day campaigning knowing he is behind despite all his bluster of doing incredibly well everywhere.
He has the harder task to win but how often have the words extraordinary, unprecedented and incredible been used in this election campaign?
Want to bet we won't use them again?
Source: Al Jazeera News