An msnbc.com guide to presidential polls

After all, it is partly Barack Obama’s and John McCain’s own polling (for which they pay dearly) that convinced them to make visits in recent days to states where they think it’s still worth competing: Pennsylvania, Indiana, New Hampshire, Ohio, Nevada, Virginia, Florida, Missouri, and North Carolina.

You won’t find McCain campaigning in California or Obama in Texas — and yet Californians and Texans are included in national samples.

The Bradley-Wilder Effect
Named for the historic gubernatorial campaigns of Tom Bradley in California in 1982 and Doug Wilder in Virginia in 1989, the theory speaks to the possible inaccuracy of polling about African-American candidates.

The idea goes that some survey respondents may not be truthful about their feelings on race during polls. Afraid to be honest or sound bigoted, they claim to support the minority candidate when they actually do not.

According to a recent paper from Harvard post-doctoral fellow Daniel Hopkins, this apparent polling bias has largely disappeared over the past 10 years.

But journalist and former Democratic campaign aide William Bradley, who worked on the Tom Bradley campaign, wrote last week on The Huffington Post website that "the problem lay not with significant numbers of voters lying to pollsters, but with the nature of the (Bradley) campaign itself.”

Tom Bradley, the Democratic mayor of Los Angeles, ran a somewhat complacent campaign and lost to Republican George Deukmejian.

As the polls closed on election night, a Field Poll projection based on exit poll interviews projected that Bradley would easily beat Deukmejian. But he lost.

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But William Bradley notes, “The Field Poll made two big projections based on its exit polling that fateful November night. Bradley as the next governor of California. And (Jerry) Brown as the next U.S. senator. But Brown lost, too.”

Recent examples
Polling can often be an accurate snapshot of Election Day outcomes. Other times, it can fail — miserably.

The greatest example in this election cycle happened in New Hampshire, in the days leading up to that state’s primary.

Surveys had Obama in the clear lead. But in the end, Hillary Clinton won the state, 39 to 36 percent. Some possible reasons for the surprise upset include last-minute Clinton traction, a loss in Iowa that turned her into the underdog candidate, and the contrarian nature of some voters.

Another polling dispute was raised just a few weeks ago, when an NBC News/Wall Street Journal poll showed Obama with a slight lead over McCain, 48 to 46 percent.

Another large national poll was released on the same day, showing Obama with a much larger lead. The McCain campaign argued the Democratic respondents in that poll outweighed Republicans by 16 points.

By comparison, this NBC News/Wall Street Journal poll had Democrats leading in party identification by just seven points.

More important than any single sample is a consistent trend among several samples.

Look, for instance, at the Pollster.com roundup of all the publicly available polling data on the presidential race in Iowa, a state which George W. Bush narrowly lost in 2000 and narrowly won in 2004.

The trend line — based on 35 samples — shows that Obama has been ahead of McCain in that state since early this year. At no point has McCain led Obama in Iowa.

Msnbc.com's Tom Curry and National Journal's Mark Blumenthal contributed to this report.

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