First, some points about the limits of polls. Random sample polling is an imprecise instrument. There's an error margin of 3 or 4 percent, and polling theory tells us that one out of 20 polls is wrong, with results outside the margin of error. Sometimes it's easy to spot such an outlier; sometimes not.
In addition, it's getting much harder for pollsters to get people to respond to interviews. The Pew Research Center reports that it's getting only 9 percent of the people it contacts to respond to its questions. That's compared to 36 percent in 1997.
Interestingly, response rates are much higher in new democracies. Americans, particularly in target states, may be getting poll fatigue. When a phone rings in New Hampshire, it might well be a pollster calling.
Are those 9 percent representative of the larger population? As that percentage declines, it seems increasingly possible that the sample is unrepresentative of the much larger voting public. One thing a poll can't tell us is the opinion of people who refuse to be polled.
Then there is the problem of cellphone-only households. In the 1930s and 1940s, pollsters conducted interviews in person because half of households had either no phone or (your grandparents can explain this) a party-line phone.
By the 1970s, phone ownership was well nigh universal, and pollsters mostly phased out in-person interviewing. Phone interviews are much cheaper and quicker.
But today the percentage of households without landline phones is increasing. Under federal law, cellphone numbers have to be hand-dialed rather than dialed by computer, as landline numbers are now even when live interviewers ask the questions.
Cellphone-only individuals tend to be younger and more Democratic than landline owners. Most pollsters are conducting a set number of interviews with cellphone-only households. But they can only guess at what percentage of the electorate they'll constitute. Oversample them, and you'll get overly Democratic results.
That, many conservatives are arguing, is what pollsters have been getting in polls this month. They point out that Mitt Romney is running ahead among Independents in many polls but trails overall.
This can only happen if Democrats have a big lead in party identification, as they did in 2008. In the exit poll then, 39 percent of voters identified themselves as Democrats and 32 percent as Republicans.
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