Four stories are at the heart of any campaign. If you understand them, you know who controls the message — and with it, perhaps the election. These stories make up what campaign strategists call the “message grid,” which has four quadrants. The first two comprise the positive stories the candidates are telling about themselves; the other two feature the negative stories each candidate is telling about the other.

In some elections, one quadrant of the grid dominates the conversation — for example, when the economy or a candidate is particularly strong or weak. Campaigns jostle for position on the grid, trying to emphasize the stories they prefer and to alter elements of the stories their opponents are effectively telling. In 2008, the stars were aligned for a new and exciting candidate to tell a story about hope and change after eight years of fear and loathing, skillfully turning his “different-ness” into an asset.

But 2012 is not 2008. This year, the stories President Obama and Mitt Romney can tell about themselves are just not that compelling. In contrast, the stories they have to tell about each other are far more powerful. As we put the theater of the conventions behind us and move into the homestretch of the campaign, that simple fact — along with the omnipresence of outside groups flush with unchecked money and unchecked facts — means we can expect the nastiest two months of attack ads in modern American history.

The stories in the first two quadrants, the positive ones the candidates tell about themselves, are usually the stuff of biography ads, warm-and-fuzzy convention videos and “humanizing” testimonials from spouses and elderly parents. These stories seek to establish a relationship with voters, leaving them with the sense that the candidate shares their values, understands people like them and is the right person for the times.

The best of these stories weave together a candidate’s life and values with the lives and values of everyday people. In 1992, when Americans were anxious about a faltering economy, Bill Clinton stepped in as the “man from Hope.” His life story — a poor boy from a small town in Arkansas whose father died in a car accident before he was born but who made good despite adversity — suggested that anyone could make it in America.

Yet stories in this positive space don’t have to be so personal to be effective. Ronald Reagan never focused strongly on his life history. But the tale his reelection campaign told in his “Morning in America” ad — of a nation that was moving again, strong again and proud again — was one of the most powerful in recent memory. Like Clinton’s story, it provided the kind of hope and enthusiasm that captivates an electorate, and the positive emotion that propels a campaign forward.

The stories in the remaining two quadrants in the grid are less inspiring but just as important. They reinforce or supply voters’ anxieties or misgivings about an opposing candidate, motivators that can be just as potent as enthusiasm and hope. We often associate these negative stories with the underbelly of politics — as when associates of George H.W. Bush used racial politics against Gov. Michael Dukakis in 1988 with the infamous “Willie Horton” ad, which told the story of a convicted black murderer who raped a white woman while out of prison on a furlough program approved by Dukakis.

As much as we may think of the first two quadrants as the “good” ones, and the last two as negative and destructive, retelling your opponent’s story to stoke voters’ negative emotions can be essential to an effective — and ethical — political campaign. In 2008, for example, the Republican nominee, Sen. John McCain, pulled ahead of Obama for the first time at the beginning of September, in part because Obama sought to run almost entirely in one quadrant of the grid — telling his own story and rarely mentioning the name of his Republican predecessor, George W. Bush. Although that might seem virtuous, it would probably have seemed less so if McCain had won the election and continued many of the policies that destroyed the economy. Not until Obama and his allies went negative and began painting his opponent as “McSame as Bush” did Obama pull back ahead, where he’d stay through Election Day.

In the 2012 race, the big themes and stories of the campaign have already become clear. In these final eight weeks of campaigning and advertising, and during the presidential and vice presidential debates, we will see both campaigns relentlessly hammering home their central stories in the message grid:

Obama on Obama

Over the past few months, as the Occupy movement brought debates over inequality and “the 1 percent vs. the 99 percent” into the national conversation, Obama has recast himself as a populist, emphasizing that he stands with the middle class, whereas Romney stands on it. This is a smart strategy. It frames the election as a choice, not a referendum on an economy that remains bleak nearly four years into Obama’s presidency. It also brings him back into what has been the mainstream of Democratic values ever since Franklin Roosevelt remade the Democrats as the party of working- and middle-class families. And it capitalizes on the populist rage that has energized the tea party movement since 2010, an anger that Democrats allowed Republicans to own.

But Obama has to walk a tightrope in telling this story, one he didn’t face in 2008, when in many ways the very fact of his candidacy was the story. Although he wants to emphasize the progress we’ve made since the Great Recession bequeathed to him by Bush and the Republicans, most Americans remain stressed to the breaking point and pessimistic after years of struggle. They aren’t buying any message suggesting that happy days are here again. With unemployment standing stubbornly above 8 percent, this is not a year when an incumbent wants to run on his economic achievements. The best summary Obama has of his accomplishments — domestic and international — is one that sticks: General Motors is alive, and Osama bin Laden is dead. That says it all.

Romney on Romney

While Obama has probably done as well he can with a relatively weak story about himself, Romney has proved far worse at conveying a positive story, even when he has one to tell. The story he wants to tell is that, as a successful businessman, he gets how the economy works and how to create jobs; that unlike Obama, who has had nearly four years to repair a failing economy, Romney understands how to get America working again. This is a strong story to offer voters desperate to hear that someone knows how to put an end to their daily struggles.

Rep. Paul Ryan, the vice presidential nominee, argued persuasively at the Republican convention that Americans generally view business success as something to celebrate, not attack. By extolling his business and management experience, Romney is suggesting an analogy, one most Americans believe, between running a business and running a government.

Like Obama’s, however, Romney’s narrative is fraught with dangers. First and foremost, while Americans are generally friendly to business, that friendship is strained after years of outsourcing, skyrocketing executive compensation coupled with plummeting wages for workers, and outrage that big business is writing the rules in Washington. Second, Romney shouldn’t need to liken executive experience in business with executive experience in government. As the former head of Bain Capital and the former governor of Massachusetts, he already has both. He’s also run a public-private partnership, the Olympics, and Americans across the political spectrum want to see government and business working together to foster prosperity.

But to win the Republican nomination, he had to hide his record as a moderate leader of a progressive state and disavow his signature achievement as governor: a health-care plan that in many respects was the prototype for Obamacare. That’s why we’ve heard so little in Romney’s “positive” story about his years in public service.

What should be clear is that both candidates are holding very weak hands when it comes to the first two quadrants of the message grid of 2012. The best Obama can say is, “It could have been worse.” And the best Romney can say is, “It could have been Newt.”

Obama on Romney

Obama’s story about his opponent is borrowed from the one Sen. Ted Kennedy offered in his 1994 Senate race against Romney and from the one Newt Gingrich retold so well during the GOP primaries: that Romney is a “vulture capitalist,” a man whose idea of a successful business model is to pick at the carcasses of moribund companies or to turn healthy companies that hire Americans into tottering ones that lay them off. According to this story, Romney understands employment so well because he knows how to end it — or ship it overseas.

“Now you have a choice,” Obama told the Democratic convention during his speech accepting the party’s nomination. “We can give more tax breaks to corporations that ship jobs overseas, or we can start rewarding companies that open new plants and train new workers and create new jobs here, in the United States of America.”

In part because Team Obama has made the case so effectively, and in part because Romney and his strategists seem to have the instincts of political animals that long ago went extinct, Obama has succeeded in making this the central story of the campaign — far more central than his story about himself. If he is able to keep this up, he is headed for reelection.

He has offered other stories as well, describing Romney as a flip-flopper who has been on every side of every issue, and even pulling out the brilliant line used by Kennedy against his Senate rival: Whereas Kennedy was pro-choice, Romney was multiple-choice. The president’s campaign has also used Romney’s pandering to the right wing of his party on social issues to paint him as an extremist leading a party of misogynists.

Romney on Obama

Only at the GOP convention did Romney really begin articulating the story he should have been telling from the start. Obama has made the election a referendum on his challenger, a great strategy in tough economic times. But Romney’s message is that the president has had four years to fix the economy, and he just can’t do it. Although he and Ryan didn’t always get their facts straight, they did get their stories straight: that more than 20 million Americans still can’t find full-time work or any work at all, and that their kids need to be able to stay on their parents’ health insurance until they’re 26 because so many of them can’t find jobs.

“This president can ask us to be patient,” Romney said during his speech accepting the Republican presidential nomination. “This president can tell us it was someone else’s fault. This president can tell us that the next four years, he’ll get it right. But this president cannot tell us that you’re better off today than when he took office.”

If there’s a message that can defeat Obama, this is it. It not only strikes at the president’s greatest point of vulnerability, it creates a link between Romney and the icon of the conservative movement, Ronald Reagan, who used the “Are you better off?” line so potently against Jimmy Carter. It also resonates with the two-thirds of Americans who believe that the country is on the wrong track. In his brilliant address to the Democratic convention Wednesday, Bill Clinton defused this attack, stating categorically that neither he nor any of his predecessors could have fixed such a broken economy in four years. But I suspect that most Americans were less convinced that someone with a clearer vision and greater capacity to lead couldn’t do better. Whether Romney fits that description is the key question for undecided voters.

These are the stories we can expect to hear for the next two months. If this campaign hasn’t focused enough on the negative quadrants of the grid already, we can expect unbridled negativity from here on out. Sure, we’ll hear ads attempting to “humanize” Romney or emphasize the accomplishments of a president whose campaign of hope and change in 2008 has morphed into a campaign of search and destroy. But between a blitz of attack ads courtesy of the Supreme Court’s decision in Citizens United, and a citizenry that is anything but united, we’re going to see only half a grid — the negative half.

The prospect of an unrelenting campaign of negativity — and the very notion of the message grid — is not an accident of American politics. It capitalizes on a counterintuitive fact about the human brain, one that psychologists and neuroscientists understand as well as the best political strategist: Positive and negative feelings are not just opposites. The neural circuitry that produces feelings such as enthusiasm and disgust is almost entirely distinct. Nothing in our brains prevents us from associating diametrically opposite feelings with the same person — a point driven home by polls showing that most voters find Obama more likable than Romney but feel more favorably about Romney’s capacity to handle the economy.

The implications for our politics are profound. The ads or stories that drive up one candidate’s positives may not be the same ads or stories that drive up the opposing candidate’s negatives. And you don’t win an election with half a brain.

Drew Westen is a professor of psychology at Emory University and founder of Westen Strategies. He is the author of “The Political Brain: The Role of Emotion in Deciding the Fate of the Nation” and the forthcoming “What’s Left?”

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