With polls drawing increased attention in the closing weeks of the presidential race, perhaps it is no surprise that when supporters of one candidate do not like the numbers they are seeing, they tend to blame the messenger.

In 2004, Democratic Web sites were convinced that the polls were biased for President George W. Bush, saying they showed an implausible gain in the number of voters identifying as Republicans. But in fact, the polls were very near the actual result.

Mr. Bush defeated John Kerry by 2.5 percentage points, slightly better than the one- or two-point lead that he had on average in the final polls. Surveys of voters leaving polling places that year found an equal number of voters describing themselves as Democrats and Republicans, also close to what the polls had predicted.

Since President Obama gained ground in the polls after the Democratic National Convention, it has been the Republicans’ turn to make the same accusations of bias. Some have said the polls are “oversampling” Democrats and producing results that are biased in Mr. Obama’s favor. One Web site, unskewedpolls.com, contends that even Fox News is part of the racket in what it says is a “trend of skewed polls that oversample Democratic voters to produce results favorable for the president.”