Much of what we think we know about these key states has been knocked down by political science research.
Swing states: Pundits love to talk about them, and candidates lavish attention on them. Sometimes it seems that the nominees are running for president of the United States of Ohio, Pennsylvania and Florida, and that the rest of us are just spectators.
But much of what we think we know about these key states, which switch party allegiances with some frequency, has been knocked down by political science research - and sometimes, by recent history. Here are a few misperceptions about these in-demand states.
1. Swing-state polls are the key to predicting the winner.
In fact, the opposite is true, especially this far from November. Generally, elections are determined by a "uniform swing." That is, if the Republican candidate does a little better overall, then he's going to do a little better in close states such as Ohio and Nevada, too.
So even though the candidates will spend most of their time and money in the states they expect to matter most, it won't make much difference.
Any candidate who wins the popular vote by at least three percentage points is certain to win the electoral college, and any candidate who wins the popular vote by as much as a full percentage point is overwhelmingly likely to win the electoral college. So the best way to follow the election is to read the national polling averages.
National polls have a key advantage: There are a lot more of them, so we're less likely to be fooled by the occasional outlier. And the frequency of national polls, conducted by the same handful of firms, means informed readers can catch any obvious partisan tilts in the results and interpret them accordingly.
Granted, political junkies like me won't be able to stop themselves from peeking at what the Des Moines Register thinks is happening in the Hawkeye State. But if we're smart, we'll look at the national polls to find out what's really going on.
2. A vice presidential candidate should appeal to key groups in swing states.
We hear this every election cycle. The National Journal's latest Veepstakes rankings, for example, say that former Minnesota Gov. Tim Pawlenty makes sense as a possible running mate for Mitt Romney because "in an election that could be decided by Rustbelt battlegrounds, it couldn't hurt to have a guy capable of matching VP Biden's blue-collar appeal."
This sort of thinking is probably what led to John McCain's selection of Sarah Palin in 2008; his campaign thought she would neutralize Barack Obama's advantage among female voters. And supposedly, Joe Biden was picked to help Obama with white working-class voters in the swing states of Ohio and Pennsylvania. The problem is that there's no evidence that vice presidential candidates have that kind of impact.
The exception, research has found, is that a popular running mate might help by a couple of points in his or her home state. But even if a candidate knows what the swing states are, it's a lot harder to figure out where, exactly, the campaign could most use that two-point boost. And home-state popularity isn't transferable. So whatever folks in Duluth or St. Paul might think of Pawlenty, he might not appeal to voters in Dayton.
3. Ignore the national economy, and focus on swing-state economies.
Ever since political scientists showed that the economy is a major factor in presidential elections, they have struggled to determine what exactly that boils down to. Is it voters' personal experience?
What their friends and neighbors believe? The answer matters a lot. If the local economy is the deciding factor, then it would make sense for the candidates to focus on how the economy is doing in, say, Dade County, Fla., or Hamilton County, Ohio.
It turns out, however, that impressions of the national economy are what really move votes. As one recent study of voting and the economy concluded: "Evidently, voters believe the president has little effect on their local economy, and they do not form their evaluation of the national economy based on surrounding conditions. . . . People form their opinions of the national economy based on non-local factors, such as the national media."
4. Once a swing state, always a swing state.
It's true that some states will perpetually be competitive, but over time, some experience significant changes. West Virginia, for instance, went from being one of the strongest Democratic states in 1980 to being one of the strongest Republican states now. It's very hard to know in advance, certainly until the last few weeks of the campaign, what the key swing states - the ones that will truly determine the winner - will turn out to be.