Notwithstanding a slight narrowing in the past week, opinion polls are leaving little room for doubt as to who will occupy the White House come January. Real Clear Politics informs us that Barack Obama has led John McCain at 52 successive national polls going back to September 26, by margins ranging from 1 per cent to 15 per cent and averaging just below 7 per cent.

The decisive swing states have been polled to within an inch of their lives in the past week, and every one tells the same story. Seven polls conducted in Pennsylvania on Sunday have been published in recent days, and the smallest of Obama’s leads is 7 per cent. There were also four polls each from Ohio and Florida, both of which have shown signs of life for McCain at different points of the campaign, and here too Obama has led in every one.

That leaves the question of whether there is a systematic inaccuracy at work in the polls, and here’s where things get interesting. For all the cynicism the subject engenders, opinion polls in Australia generally prove to be highly accurate, in part because compulsory voting neutralises the troublesome variable of voter turnout. In the US, voluntary voting combined with high levels of cultural and regional diversity raise question marks over even the most emphatic of polling trends. A number of items of conventional wisdom are at large to suggest they might not be telling the full story, one way or another:

The Bradley effect. A paper by Daniel Hopkins of Harvard University examines the popular notion that polls overrate the performance of black candidates in biracial contests due to white voters’ reluctance to appear illiberal when interrogated by pollsters. Hopkins finds the effect was a serious factor into the 1980s, most famously when black Democratic candidate Tom Bradley failed to win the Californian gubernatorial election in 1982, but has ceased to be so.

Pew Research charts a corresponding decline in the number of respondents willing to admit they would not vote for a black candidate, from 16 per cent in 1984 to 6 per cent. Hopkins notes a very sudden decline in the Bradley effect “at about the time that welfare reform silenced one critical, racialized issue”. However, it might be conjectured that Obama also has Islamophobia to contend with, the effect of which has never been measured.

The reverse Bradley effect. Strictly speaking, a “reverse Bradley effect” would involve voters telling pollsters they were voting for McCain or were undecided when they were in fact set on Obama. Far more likely is that turnout of black voters is being underestimated in pollsters’ determinations of “likely voters”, which in many cases go on whether they voted last time rather than what they say they will do this time.

Whatever methods are being used to account for the certainty of higher black turnout, they are likely to be overly conservative: when a pollster is required to explain inaccuracy after the event, “I was going on past experience” makes for a more professional sounding excuse than “I made a wrong guess”. One eyebrow-raising example was the highly reputable SurveyUSA’s most recent poll of Pennsylvania, which had 10 per cent of black voters among its overall sample — whereas the Joint Center for Political and Economic Studies tells us it was 13 per cent in 2004.

Nate Silver at FiveThirtyEight provides support for this and related impressions in taking to task pollsters who have gaps of 4 to 6 per cent between results for “registered” and “likely” voters.

The late Republican surge. There is a widespread view that Republican candidates tend to come home strong in the last week or two of campaigning, which certainly seemed to be the case on both occasions when Bill Clinton faced election. However, there was little evidence for it in 2000 and 2004, when if anything the late polls trended slightly to the Democrats.

Front-runner decline. The aforementioned paper by Daniel Hopkins provides ample evidence for the proposition that polls “typically overstate support for front-runners”. Using a large number of observations from contests for Senator and Governor over recent decades, it appears a candidate like McCain who is polling in the low 40s is likely to do about 2 per cent better on the big day, whereas a candidate like Obama on about 50 per cent is probably being represented accurately.

Late advertising. The cashed-up Obama campaign has reportedly been fielding as many as seven commercials for every one aired by McCain in the past week, culminating in today’s nationally televised 30-minute “informercial”.

My guess is that point one will be comfortably countered by point two, while point three is worth little if anything. All that leaves McCain with is the narrowing indicated by point three, which might prove at the higher end of market expectations if shifts in the most recent poll results are meaningful.

This being so, I expect Obama to win the previously red states of Iowa, New Mexico, Colorado, Virginia, Ohio, Nevada and Florida, leaving him well clear of the “magic 270” with 338 electoral votes. He is also in contention in Missouri, Indiana and North Carolina, although most pundits rate McCain the favourite here.

I’m going out on a limb to predict (without confidence) that the well-oiled Democratic mobilisation campaign will bring these states home as well, pushing Obama up to 375 against 163 for McCain.