Yesterday marked one year until the 2012 presidential elections.

It's easy to underestimate how far away that is. At a similar point before the 2008 election, Rudy Giuliani and Hillary Clinton were viewed as the most likely candidates. One year before the 2004 election, Howard Dean led the polls for the Democratic Party. A year before the 1984 election, Ronald Reagan's re-election chances looked dim as the economy drooped. No one knows what might happen a year before an election.

But what is fairly certain, as the Financial Times wrote nearly a half-century ago, is that, "All political history shows that the standing of a Government and its ability to hold the confidence of the electorate at a General Election depend on the success of its economic policy." Winston Churchill once echoed a similar message: "It is no longer a case of one party fighting another, nor of one set of politicians scoring off another. It is the case of successive governments facing economic problems and being judged by their success or failure in the duel."

For decades, it's been clear that one of the most important factors in any election is the state of the economy. Reagan rode this idea in the 1980 election with the campaign question, "Are you better off now than you were four years ago?" Bill Clinton followed up in 1992 with the now-ubiquitous and catchy phrase, "It's the economy, stupid."

That might be one of the only campaign slogans that has real, verifiable proof to it. As Yale economist Ray Fair has shown, changes in unemployment and economic output a year before elections have a big effect on election results. Every 1% rise in economic output in the year before elections gives incumbent candidates an extra 1.2% of the electoral vote, Fair found. Every 1% rise in the unemployment rate cost incumbents 2.3% of the vote. Indeed, two things surged last month: the stock market, and President Barack Obama's approval numbers.

As Harvard historian Niall Ferguson wrote in his book The Cash Nexus:

"It has become an axiom of modern politics that there is a causal relationship between economics and government popularity: to be precise, that the performance of the economy has a direct bearing on the electoral success of an incumbent government. A good illustration of this new economic determinism was the widespread explanation of the failure to impeach President Clinton for perjury and the obstruction of justice in connection with his numerous sexual misdemeanors. By February 1999 a majority of Americans believed Clinton was guilty of the charges against him, but only a small minority wanted him to resign as president. According to Senator Robert Byrd -- and many other commentators -- the explanation was simple: 'No president will ever be removed when the economy is at record highs. People are voting with their wallets in answering polls.'"

On the contrary, Ferguson writes, in the year before Richard Nixon resigned in 1974 with approval ratings under 30%, "unemployment rose by almost 1 million and the inflation rate doubled ... on Wall Street the stock market fell by a third." Voters won't find forgiveness when they can't find a job. What's more, voters are usually more enthusiastic about punishing a candidate when things are poor than they are about rewarding a candidate when things are good.

That's fascinating when you consider today's economy. On one hand, unemployment is at a generational high, real wage growth is stagnant, and the stock market is where it was 10 years ago. Only two presidents over the past 30 years haven't been granted a second term -- Jimmy Carter and George H.W. Bush. Both presided over a dismal economy on Election Day. On the other hand, depressed economies like we have today are the kind of environments that set the stage for quick, spiking rebounds, as happened in the early 1980s. A year and a half before the 1984 presidential election, the unemployment rate stood at 10.5%, the highest since the Great Depression. By election day, it was 7.2% and dropping almost every month, which helped usher Ronald Reagan into a landslide victory. Very few pundits surveying the scene in 1983 saw that economic boom, and the political implications it would have, coming.

Which is almost always how it works. In a recent article in BusinessWeek, Charles Kenny cited an International Monetary Fund study of 60 worldwide recessions. "A grand total of two of those 60 were predicted by forecasters a year before they happened -- which means the other 58 took economists by surprise," Kenny wrote. Statistician Nate Silver recently wrote in The New York Times about how awful economic forecasts are for those attempting to use them to predict the outcome of elections:

"[E]conomic forecasts are not very good. In fact, they are completely terrible. In November 1995, economists expected the economy to grow at 2.6 percent the next year; it actually zoomed upward by 4.4 percent. In November 2007, they expected it to grow at 2.5 percent, but it shrank by 3.3 percent, as the effects of the global financial crisis became manifest. Frighteningly enough, the margin of error on an economic forecast made a year in advance is about plus or minus 4 percent of G.D.P. Advance forecasts of election results must account for this uncertainty."

This all leads to an unsatisfying but honest reality: The key to the 2012 elections is the economy, but no one -- absolutely no one -- knows what the economy will do over the next year.

That won't, however, deter us from making predictions, forecasting results, and debating the significance of the upcoming election. How could it? Next year's election seems more important than ever, given the weak economy and exploding deficits.  

But as Washington lobbyist Andrew Lowenthal noted last year: "Every election I've ever been involved with has been 'the most important election in history.' At some point it's not. It's just the path of history." Same story, different day.

Fool contributor Morgan Housel doesn't own shares in any of the companies mentioned in this article. Follow him on Twitter @TMFHousel. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.