Photograph by Justin Sullivan/Getty

Last weekend, the Sunday news shows were abuzz about the growing consensus in the mainstream media that the G.O.P. holds a slight advantage in the battle for the Senate. This forecast has come from the New York Times’ The Upshot, the Washington Post’s The Monkey Cage, and FiveThirtyEight’s Nate Silver, the nerd king of statistical analysis. All of these predictions have one thing in common: they are based on more than pure polling data. By considering other factors, these data journalists are putting their thumbs on the scale—lightly, but with consequential effects.

Here is a table of current polling data for the key Senate races, along with win probabilities issued by various handicappers:*

The first column of numbers lists the median lead by either candidate in the recent polls.* The next three columns show Democratic win probabilities from sites that only rely on polling data: the Princeton Election Consortium, which I founded; the Huffington Post’s Pollster; and Daily Kos’s Poll Explorer. Win probabilities higher than fifty-five per cent are colored blue if Democrats are favored, and red if Republicans are. The rationale for these calculations is straightforward: if a candidate is leading now, he or she is likely to win in November. The discrepancies in these numbers come from the slightly different rules each organization uses to average the poll results.

The colors change once we reach the Post, FiveThirtyEight, and the Times: a sea of red starts to appear. Compared with the average probabilities from the first group, the Post’s figures come the closest—within two percentage points—but they still favor the Republican candidate. The Times leans farther right, by an average of six percentage points, and FiveThirtyEight veers the hardest to the right, favoring the Republican candidate in nearly every race by an average of twelve percentage points.

In addition to polling data, these analysts are taking into account “fundamentals”—factors that supposedly capture the state of the political playing field—like incumbency, campaign funding, prior experience, and President Obama’s job-approval rating.

Fundamentals can be useful when there are no polls to reference. But polls, when they are available, capture public opinion much better than a model does. In 2012, on Election Eve, for example, the Princeton Election Consortium relied on polls alone to predict every single Senate race correctly, while Silver, who used a polls-plus-fundamentals approach, called two races incorrectly, missing Heidi Heitkamp’s victory, in North Dakota, and Jon Tester’s, in Montana.

The Princeton Election Consortium generates a poll-based snapshot in which the win/lose probabilities in all races are combined to generate a distribution of all possible outcomes. The average of all outcomes, based on today’s polls, is 50.5 Democratic and Independent seats (two Independents, Bernie Sanders and Angus King, currently caucus with the Democrats).

Simplicity, Simplicity, Simplicity!

I did not always appreciate the importance of sticking closely to polling data. I first started analyzing polls during the 2004 Presidential campaign, in which John Kerry and George W. Bush traded the Electoral College lead three times between June and November. An October calculation based purely on polls suggested that Bush would win. However, I added an extra assumption: that undecided voters would break by two percentage points toward Kerry. On Election Day, the president of my university e-mailed me asking for my final prediction. I told her, with confidence, that it would be Kerry. It was a humbling mistake.

Because polls have better predictive value than fundamentals do, it would seem prudent to ask what an unadulterated poll-based snapshot of the Senate race looks like. Today, it looks like this:

Based on this calculation, if the elections were held today, Democrats and Independents would control the chamber with an eighty-per-cent probability. (The green section accounts for Greg Orman, the Independent candidate in Kansas, who would provide the fiftieth vote. Orman has said that he would caucus with the majority, that he would caucus with the other Independents, and that he wants to break the Senate gridlock. For this histogram, I have graphed him as caucusing with the Democrats.)

But can a snapshot of today’s polls really tell us that much about an election held eight weeks from now? As it turns out, it might. A poll-based snapshot moves up and down, like the price of a stock. That movement can show us the range of the most likely outcomes for Election Day. The chart below displays those ups and downs. On the right is a zone of highest probability, drawn out in much in the same style as a hurricane strike zone on a weather map. This area indicates where the campaign is most likely to land.