WASHINGTON (AFP) - For John McCain to confound the pundits and upset favourite Barack Obama in Tuesday's election, he must navigate a perilous path on which a single slip could hand his rival the White House. McCain is battling to hold onto many of the key states, which pushed Republican President George W. Bush to re-election in 2004 " and in some scenarios the loss of just one major battleground could mean a president Obama. By contrast, the Democratic nominee clearly has a much simpler task " hold onto the states won by his 2004 counterpart John Kerry and add a big state or several smaller ones to get him to the winning post of 270 electoral votes. Facing almost universally bad news in opinion polls, McCain's campaign hopes for an upset that would rank as one of the most dramatic eleventh-hour turnarounds in American political history. Such an outcome would cast deep doubts over the methodology of pollsters and expose much of the US journalistic establishment to ridicule. Obama led the latest NBC/Wall Street Journal survey by eight points, the Rasmussen daily tracking poll by six points and the USA Today/Gallup survey by 11 points. He led poll averages by independent website RealClearPolitics in all the battlegrounds save North Carolina, now a tie and held narrow leads in Indiana and Missouri " which are normally Republican territory. Under the US system, the president is chosen on the base of electoral votes allocated to each state. Large states get the most votes " California for instance has 55 while those with small populations get as few as a handful. In most cases, the winner of the popular vote in each state gets all the electoral votes. An electoral map put together on the basis of poll averages by the site on Monday had Obama leading by 278 electoral votes to 132, with 128 still unallocated in toss-up states. Obama's campaign set out to give their candidate multiple routes to the White House on the US political map. "Our number one strategic goal was to have a big playing field," said Obama campaign manager David Plouffe on Fox News Sunday. "We did not want to wake up on the morning of November 4th waiting for one state. We wanted a lot of different ways to win this election." Obama already leads the polls in Kerry states, so he needs just one big state, or a combination of smaller states to get him over the top. Simply winning the Kerry states and Ohio's 20 electoral votes would get the job done, and give Obama 272 electoral votes. The Kerry states plus Florida (27 electoral votes) which Democrats narrowly lost in 2000 and 2004 would also do the trick. The Democratic nominee could also chose a more westerly strategy, picking up the Bush state of Iowa (seven electoral votes) where he is heavily favored and Colorado (nine electoral votes) plus one other of New Mexico (five electoral votes) and Nevada (five electoral votes). Obama is also leading the polls in normally Republican Virginia, is heavily pressing McCain in North Carolina, Indiana and Missouri, so he has many possible paths to victory. McCain however is in a much more difficult position. Given that it is unlikely the Republican will be able to defend all the Bush states in 2004, according to polls in a tough Republican year, he needs to flip at least one big Democratic state " likely his only realistic option is Pennsylvania (20 electoral votes). The Republican nominee however faces a tough fight to hang on to Virginia, Iowa, Missouri, North Carolina, Florida, Nevada, Colorado and New Mexico " all of which went to Bush in 2004. The loss of one of those big states to Obama " Florida or Ohio for instance, could spell doom for McCain. The loss of Virginia, and one other smaller state from the Bush map, like Colorado for instance, could also mean an Obama presidency. McCain hopes Obama's final totals do not reflect his strength in national and swing state polling, and his campaign argues the race has trended clearly towards him in battleground states in recent days. Some analysts also think that Obama's race may mean he underpolls opinion surveys as some people may not want to tell pollsters they are not willing to vote for an African-American. Theoretically, Obama could still rack up huge totals in national polls by maximizing support in Democratic states like California and New York, and still face a tough fight against McCain for electoral votes in the battlegrounds. But it seems statistically highly unlikely he could win the popular vote millions but still lose the electoral college.