With Romney trailing by a clear margin, a frontal assault on the accuracy of polling has begun. Some of the skepticism is understandable, but other elements are brazenly self-serving. I'll be assessing the critiques of the polls over the next few days, but any debate about the accuracy of the polls must start by remembering a central point: The polls are usually pretty good.

Forget about LOESS trend-lines, demographic regressions, or House Effects and just consider the surprising accuracy of a polling average that can be calculated by any sixth grader with a calculator and access to the Internet. In 2004 and 2008, the RealClearPolitics average ended with Bush leading by 1.5 points; he ended up winning by 2.4 points. In 2008, Obama led by 7.6 points; on Election Day he won by 7.2 points. Pretty good, right?

The battleground state polls were also accurate. In 2004, the RCP average only missed Wisconsin, but Bush entered Election Day with just a .9 point lead and Kerry only won by .4 points, so it’s hard to characterize that as a real failure. In 2008, the RCP average only missed Indiana and North Carolina, the two closest states won by Obama. Again, the average got the basics of a close race right: McCain entered Election Day with a .4 point lead in North Carolina, he would lose by .3 points; Obama trailed by 1.4 points in Indiana, he would eventually win by just 1 point. There are examples of more substantial errors, like when the RCP average showed Kerry within one point of Bush in Florida, even though Bush would ultimately prevail by 5 points. But on average, the state averages were off by just 2.8 points in 2008 and 1.9 points in 2004—not perfect, but more than good enough for our purposes.

Polls are typically less accurate in judging races further down the ballot, but they still do pretty well. They might also be less accurate during off-year elections, or when the contested races are in deep red or blue states. Of the 24 closely contested gubernatorial, senate, or presidential contests where the polls exhibited a Republican bias of 3 points or more, just three were in states carried by John McCain in 2008, and two were West Virginia and Kentucky, states where Democrats hold a large advantage in party-ID. Conversely, of the 14 gubernatorial, senate, or presidential contests where the polls tilted Democratic by 3 points or more, just four were in states carried by John Kerry, and only Rhode Island was non-competitive at the presidential level over the last decade.

But the polls tend to do quite well in close contests in the battleground states, where swing voters seem to split evenly between the candidates between the final polls and Election Day. If there’s one exception, it might be the competitive western states, like Nevada, Colorado, and New Mexico. In 11 competitive contests in those three states, the polls underestimated the eventual Democratic performance in every instance, including the two upset Democratic victories. Some have suggested this is due to difficulties in polling Latino voters, but I’ll shy away from explaining the error and simply observe its existence.