In a few short weeks, Alan Abramowitz will predict whether Barack Obama or Mitt Romney will win the popular vote for president — and he’s almost certain to be right.

Abramowitz, a political science professor at Emory University, has correctly called every popular-vote winner since he began forecasting elections in 1992. He has come within two percentage points, on average, in predicting the winner’s share of the vote in those elections, too, including Al Gore’s super-squeaky majority in 2000.

How can anyone accurately estimate the outcome of an election more than three months ahead of time — before the conventions, the debates, and the twists and turns of the fall campaign? Primarily because Abramowitz’s forecasting model disregards the fall campaign altogether. His method acknowledges something that political operatives, journalists and candidates rarely do: Presidential campaigns don’t matter much in determining winners and losers.

Despite all the noise from the campaign trail — from the onslaught of TV ads to the daily rallies to the frenzied news coverage — factors beyond either candidate’s control largely determine the result, according to this school of thought. So much is already baked into a presidential contest that even the best managed and most effective campaign (or the most incompetent one) can’t move the needle too far.

This idea has been around since at least the 1940s and has been so thoroughly studied that it has its own wonky name, the Minimal Effects Model. Simply stated, the model says that presidential campaigns have a highly limited effect on how people vote. Because of partisan loyalties and other structural factors, millions of voters have made up their minds long before the most intense electioneering begins, leaving only a disengaged few for the candidates to persuade.

“When you’re in the middle of a campaign, there’s a tendency for people, especially the media, to overestimate the importance of certain events,” Abramowitz says. These include high-profile gaffes, vice presidential selections, controversial ads and other moments that capture so much attention.

Except, he adds, “those things have no measurable impact [on voters’ decisions]. The media are interested in getting people’s attention, but a lot of the stories you read or see are focusing on things that are trivial. The way campaigns play out is largely determined by fundamentals.”

In his case, the “fundamentals” are broad measures of the electorate’s happiness or dissatisfaction. Abramowitz plugs just three variables into his forecasting model — the president’s approval rating in mid-year, economic growth in the second quarter, and whether either party is seeking a third consecutive term (he gives the incumbent party’s candidate a bump if the answer is no).

Other forecasters, such as James E. Campbell of the State University of New York at Buffalo, have achieved results similar to Abramowitz’s by considering a sequence of Gallup polls several months before Election Day. Yale economist Ray C. Fair, the author of “Predicting Presidential Elections and Other Things,” has accurately modeled 21 of the 24 elections since 1916 with a method that combines economic growth, inflation and the effect of incumbency.

These election-prediction models discount or even ignore the things that political junkies obsess over. Remember the brief flap in June about President Obama’s comment that “the private sector is doing fine” when the unemployment rate was 8.2 percent? As Dartmouth government professor and blogger Brendan Nyhan pointed out, Obama’s approval ratings increased in the three days after that “gaffe.” Likewise, there’s no evidence that Romney adviser Eric Fehrnstrom hurt his boss when he said on CNN in March that the fall campaign would be like an “Etch-a-Sketch,” a blank slate.

Even more startling, this analysis essentially ignores the candidates themselves. “The assumption is that the major parties basically nominate reasonable candidates that are both well-funded and reasonably well organized,” Campbell says. “They have equally good pollsters, media advisers, strategists. Therefore, they tend to cancel each other out.”

By this reasoning, developments in the final months of the campaign rarely make much of a difference. Absent the start or conclusion of a war, a massive economic shock or the mythical “October surprise,” the vast majority of voters don’t zig or zag in the run-up to Election Day. According to Campbell, the candidates leading in the Gallup poll in late September have won in 14 of the past 15 elections.

Consider, for example, the last months of the 2008 campaign. The financial meltdown that fall turned what had been expected to be a close race into a relatively easy one for Obama. (Sen. John McCain’s decision to suspend his campaign briefly and return to Washington probably only intensified the electorate’s unease about the economy, if not about the Republican himself). Although some attributed Obama’s victory to a brilliantly waged campaign, the economic news in September and October, coupled with President George W. Bush’s deep unpopularity, would have been decisive for just about any Democrat four years ago, says John Sides, a political scientist at George Washington University.

Academic election-forecast models rest on the notion that campaigns don’t have much effect because there aren’t many voters left for the candidates to persuade, especially as the election approaches. Even in the worst of times, both major-party candidates can count on at least 40 percent of the vote; in the best of times, neither can expect more than 60 percent, Campbell says. “Defections” to another party are rare; some research suggests that voters change their party identification about as often as their religious affiliation, which is to say not very often.

People who describe themselves as “leaning” toward one candidate months ahead of the election overwhelmingly end up voting for that candidate. About 91 percent of Obama’s “leaners” went for him in 2008, Campbell says.

This implies that the pool of “persuadable” voters is tiny — more kiddie-size than Olympic. And the pool becomes even shallower as the few holdouts start making up their minds. The closer to the election, the fewer persuadables there are. In a Gallup/USA Today poll in May, the most recent time that Gallup asked, only 7 percent of registered voters in 12 swing states said they were undecided. That number probably is lower now.

The campaigns implicitly recognize this but rarely mention how few voters they’re actually fighting for. This year, most of Obama and Romney’s campaign activity will be concentrated in about eight swing states, where they and their allies will spend an estimated $2 billion to move a subset of a subset of the nation’s electorate, perhaps a few hundred thousand votes in all. It’s not much of an exaggeration to say that the campaigns are training their massive firepower on some guy named Larry in Ohio and somebody’s Aunt Betty in Tallahassee or Tampa.

When they’re not spinning reporters or putting out the campaign’s message of the day, political operatives do acknowledge that much is beyond their control.

“I always tell audiences that aren’t as engaged in the day-to-day [activities of the] campaign that we’re often at the mercy of the big factors,” says Kevin Madden, an adviser to Romney’s 2008 and 2012 campaigns. “I would say that the more you work on these, the more you become aware that you can’t direct everything. You’re looking at a handful of states and a couple of hundred thousand people who can move the election one way or the other.”

For the news media, the implications of this should be sobering. News from the campaign trail is valuable and worthwhile; millions of people want and need to know how the next president is comporting himself. But as Abramowitz notes, the media err, regularly and repeatedly, by reading more into ephemeral campaign events or statements than would-be voters do. Is this or that really a game-changing moment? Or is it, at best, just another thing that happened today?

Of course, none of this is going to stop anyone from campaigning for president or raising millions of dollars to run ads that won’t change many minds. Our modern campaign industrial complex isn’t shutting down anytime soon, nor should it. Campaigns do have some role in influencing the outcome. “Minimal effects,” after all, doesn’t mean no effect whatsoever.

Even true believers in the Minimal Effects Model recognize that campaigns and candidates are important, albeit at the margins. Those margins are important, even decisive. The 1 percent or 2 percent of voters in Ohio or Florida whom the campaigns are fighting over could be the difference between victory and defeat in November. Just ask President Al Gore.

Campbell and other political scientists say one job of the campaign is to motivate partisans — rallying the base — and to mobilize them on Election Day. Candidates prime voters by framing the issues in ways that will cause people to go to the polls. They also inform the electorate about who they are, detailing their positions, experiences and background. In other words, campaigns can affect what voters know and whether they vote. Although no candidate can transcend the fundamentals, he can still make his best possible case.

As for the forecast this fall, stay tuned. Abramowitz is a few weeks away from cranking up his prediction machine, so he’s not ready to call the race. But he doesn’t need a mathematical model to tell him what even the most casual observer can already see. “It’s going to be a close election,” he says.

Paul Farhi is a Washington Post staff writer.

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Biggest whoppers in campaign history

When honesty hasn’t been a winning policy

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