As Tuesday's elections near many political analysts are using polling data to make the case that Republicans are in a good position to retake the Senate, but Democrats are now questioning the polls' accuracy, saying they undercount Democratic voters.
"The polls have generally underestimated Democrats in recent years, and there are reasons to think it could happen again," Nate Cohn writes in
The New York Times.
Cohn says there is a history of such undercounting, including in 2010, when Senate polls "underestimated" Democratic candidates by an average of 3.1 percentage points.
In 2012, the pre-election polls missed the enthusiasm for President Obama in key battleground states by an average of 2 percentage points, Cohn says.
Another argument is that polling has not kept up with technology and the growing dependence on cellphones, rather than landlines, particularly by minority voters.
According to a
2012 Pew Research analysis, the response rate of a typical telephone survey in 1997 was 36 percent. In 2012, the response rate was 9 percent.
"Although response rates have decreased in landline surveys, the inclusion of cell phones — necessitated by the rapid rise of households with cell phones but no landline — has further contributed to the overall decline in response rates for telephone surveys," says the Pew study.
Alicia Caldwell of the Denver Post says the reliance on cellphones leads to an undercounting of Latino voters, a key constituency in Colorado.
She says Latino voters tend to be undercounted because they are younger and more likely to have only a cellphone, so "some (but not all) public polls have failed to give Latinos an appropriate weight in samples."
Even when pre-election polls are proven to have undercounted one side or overestimated the strength of one party, no one should conclude there's an inherent bias in the forecast, according to Democratic political strategist
Mark Mellman.
"Models based only on fundamentals — presidential popularity, economic changes, midterms vs. presidential year — tell us a lot about political outcomes. They aren’t perfect, they cannot tell us everything, but they do reveal a great deal.
"And those fundamentals exclude the choices of campaign managers and consultants, party committees and presidents," he writes in The Hill.
Nor are questions of accuracy new.
Nate Silver of FiveThirtyEight says there is a "long tradition" of the trailing party claiming that internal polls paint a different electoral picture than public polls, and that those polls do not factor in voter turnout operations.
"In 2004, some Democrats asserted that John Kerry would outperform his polls because undecided voters would break toward him. Instead, George W. Bush won by a slightly wider margin than the polls predicted. Throughout 2012, conservatives argued that the polls had a Democratic bias. The polls did have a bias — but it was a Republican one," he writes.
Silver acknowledges there can be a bias in polling, but argues it favors one party in one year, and another party the next year.
The reason, Silver says, that there is little evidence of a long-term bias toward a particular political party is that polling is not static and pollsters can make adjustments, such as contacting more voters via cellphone.
Echoing Silver's assertion that the polls might be biased in any given year is
Sean Trende of RealClearPolitics, who says that bias does not necessarily mean Democrats will retain control of the Senate.
"The only thing we can say with any sort of certainty is that in some years the polls have had a slightly pro-Republican skew, while in other years they have had a slightly Democratic skew, while in still other years they’ve had no skew.
"Moreover, even though the skew might be material, it is typically also small: enough to flip a 51-49 race but not enough to save a candidate who is down, say, 50-44," Trende writes.