The winner of the US presidential election is not always the candidate who wins the popular vote - the path to victory is to win a majority in the electoral college.
Each state has a certain number of electors who elect the president - the number depends on the state's population - and the trick is to win enough states to stack up 270 electors' votes.
The map to the right shows how the states have voted since 1948.
Below we reprint five expert projections, based partly on the latest opinion polls, which indicate how the race could end in 2008.
Each website regularly updates its map. This is what the maps looked like as polls opened on the east coast of the US.
CQ POLITICS: Obama 311, McCain 157
The Congressional Quarterly has tracked both presidential and congressional races for many years.
Its current calculations suggest that Barack Obama is ahead in two major swing states, Ohio and Virginia.
Its list of toss-up states includes not only Florida and North Carolina, but also Montana, Missouri, North Dakota and Indiana, which were considered solidly Republican until very recently.
We have slightly simplified the CQ Politics map. As well as toss-up (or, in its terminology, "no clear favourite") it has three categories of states: "safe" "favoured" and "leans". We have amalgamated the "favoured" and "leans" category. (This is why more states appear in the "leaning" category on this map than on the other four.)
REAL CLEAR POLITICS: Obama 291, McCain 132
The Real Clear Politics website aggregates election polls, and rates states by averaging the most recent polls in that state.
The most striking feature of this map is the fact that Arizona - home state of Republican presidential candidate John McCain - now appears in the toss-up category. (This change was made on 31 October).
Real Clear Politics also lists Georgia as a toss-up, in contrast with CQ, which lists both states as "favoured" for McCain.
On the other hand, Real Clear Politics has recently moved Pennsylvania (30 October) and Minnesota (3 November) from the solid Obama category, to the leaning Obama category. And on 2 November it moved Virginia and Ohio from "leaning to Obama" to the toss-up category.
The website has also recently moved South Dakota and Arkansas from the "firm" for McCain category into "leaning".
POLLSTER: Obama 311, McCain 142
Pollster.com is another site that takes polls from other sites and uses them to estimate state-by-state results. Both tracking sites now broadly agree on the shape of the race.
As of noon GMT on Saturday 1 November, Pollster had not followed Real Clear Politics in moving Arizona into the toss-up category, or Pennsylvania into the leaning Obama group.
It also sees New Mexico as solid for Obama, whereas Real Clear Politics classifies it as leaning towards Obama.
Like Real Clear Politics, Pollster also sees support for the Republican candidate weakening in South Dakota.
NEW YORK TIMES: Obama 291, McCain 163
The New York Times combines data from state polls with the judgement of its correspondents on the ground.
It regards the contest as much closer than CQ, Real Clear Politics or Pollster.com.
Like most other sites, it now suggests that support for Mr McCain is weakening in some unlikely places, such as Georgia, Montana, North Dakota and West Virginia.
CRYSTAL BALL: Obama 364, McCain 174
The website of Professor Larry Sabato, a well-known election expert from the University of Virginia, is now (since 30 October) predicting a landslide - more than 350 electoral college votes - for Barack Obama.
Professor Sabato no longer lists any states as toss-ups. He has also stopped referring to states as "leaning" to one candidate or the other. Instead he says the race in certain states is "close".
However, he lists close races in either red or blue, so we have continued to describe them as "leaning", and shaded them in the same colour as the leaning states on the other maps, to make a visual comparison easy.
Perhaps because of this change of terminology, Professor Sabato is the only one of our five experts who - instead of listing Florida and Missouri as toss-ups - sees them as tending towards Barack Obama.