Eve-of-poll voter surveys mostly point to a narrow US presidential election victory for Barack Obama. But with the contest finely balanced in a few key states, it would not take much to shift the advantage to Mitt Romney. A last-minute slip, a neglected minority voter bloc, or simply a piece of bad luck could be enough to ditch Obama and send the Republican to the White House.

Here's one possible example: Obama's unguarded suggestion in Springfield, Ohio last Friday that "voting is the best revenge" has been seized on by Republicans. Romney exploited it in a speech later that same day. "He [Obama] asked his supporters to vote – to vote for revenge. Instead, I ask the American people to vote for love of country," Romney said – and then aired a TV attack ad featuring the exchange.

"'Revenge' is not something sought by most American voters, and 2008 Obama voters who are disappointed in his performance … Voters want someone who will serve the country. The moment when Obama said "revenge" could turn out to be a pivotal moment in the campaign", said Beltway Confidential commentator Michael Barone. Obama faces plenty of other possible last-minute pitfalls as he reaches for the finishing line.

The Bradley effect

Named after a former black mayor of Los Angeles, this phenomenon involves voters telling pollsters they intend to support a black candidate, and then not doing so because of his or her ethnicity. In 2008 Obama enjoyed an 8-9% lead in most polls, but his actual margin of victory was 6%. If today's polls are overestimating his backing by a similar margin, Romney will win.

Racial prejudice, covert or otherwise, also partly explains why a majority of white males will vote against Obama. In 2008, 57% backed Republican John McCain. The anti-Obama totals among both white men and women may be higher this time, polls suggest. It is also a fact that white males prefer Republicans to Democrats, whatever their colour.

Turnout

The Democrats need a big turnout to secure victory but most estimates suggest Obama's support, particularly among younger voters, blacks and Hispanics, will fall well below 2008 levels, partly as a result of disappointment with his performance in office.

"In 2008, voters 18 to 29 went for Obama 2-1 over McCain; turnout among these young voters was the second-largest ever recorded. But in 2012, that youthful Obamamania seems to have faded. Alex Wirth of the Harvard Public Opinion Project has forecast that turnout for voters under 30 will be 34 to 40%, compared with 51% four years ago," said Fast Company's Anya Kamenetz.

Swingers

At the heart of American pollsters' electoral map lies an awkward little secret. Presidential elections are increasingly decided by an ever-smaller group of swing voters within an ever-smaller number of swing states. Nobody really knows exactly who or where these people are, and that makes for severe unpredictability. But Republicans are claiming independent voters (neither Republican nor Democrat registered) are breaking 2-1 for Romney in toss-up states and that early voting turnout shows the Republican base is more energised.

Acts of God

Religiously-inclined voters could also give Obama a scare. "I expect Mitt Romney to maximise the white religious vote … The major push Obama made in 2008 among white Catholics is completely absent this cycle … Anti-Mormon sentiment has never caught on among the Christian community to the degree some feared. And Obama's contraception and abortion message has caused a great deal of concern among religious groups, Protestant and Catholic alike," said analyst Ben Domenech.

Perfect storm

Despite suspicions about Hurricane Sandy, Obama does not control the weather. Forecasters say election day will be cold, wet and windy in the north and east – not good for the needed high Democrat turnout.

Cock-up or conspiracy?

Comparisons are already being made with the 2000 presidential election that saw George Bush installed as president after numerous recounts, legal challenges, and myriad balloting irregularities gave rise to claims of widespread fraud and rigging. Today's election is supposed to be better organised. But Americans are a sceptical lot. If the vote is as close as predicted, and the result is not immediately clear, claims that sinister forces are at work will not be long in coming.