• First, many of us understand that Barack Obama inherited a terrible predicament. We have a degree of sympathy for the man. But we have concerns, which have been growing over time, about whether he’s up to the job.

• Second, most of us are gravely concerned about the economy. We’re not certain what should be done about it, but we’re frustrated.

• Third, enough of us are prepared to vote against Obama that he could easily lose. It doesn’t mean we will, but we might if the Republican represents a credible alternative and fits within the broad political mainstream.

Each of these factors, in turn, can be quantified.

• The first factor, Americans’ performance reviews of Obama, can be measured through his approval ratings.

• The second factor, economic performance, can be measured through statistics like G.D.P.

• The third factor — essentially, the ideological positioning of the Republican candidate — is sometimes thought of as an “intangible.” But it can be measured too, and it matters a great deal.

As we get closer to the election and more data become available, we can indulge — even encourage! — greater complexity in the analysis. Figuring out how Obama is performing in individual states and how this translates to the Electoral College, for instance, requires a fair amount of attention to detail. But it is premature to do that now. Instead we should think big, and focus on the three fundamentals.

APPROVAL RATINGS

A president’s approval rating at the beginning of his third year in office has historically had very little correlation to his eventual fate. In January 1983, Reagan had an approval rating of just 37 percent, but he won in a landslide. George H. W. Bush had a 79 percent approval rating in January 1991 and was soundly defeated. But voters start to think differently about a president over the course of his third year; they view him more on the basis of his performance and less on the hopes they had for him. These perceptions are sharpened by the beginning of the opposition party’s primary campaign, which, of course, accentuates the negatives.

A president’s approval rating toward the end of his third year, therefore, has been a decent (although imperfect) predictor of his chances of victory. Reagan saw his approval rating shoot up to 51 percent in November 1983 amid the V-shaped recovery from the recession of the previous year — the first sign that he was headed for a big win. Obama’s approval rating may have rebounded by a point or two from its lows after the debt-ceiling debacle — but not by much more than that. In late October, it ranged between 40 and 46 percent in different polls and averaged about 43 percent.

There have been two presidents stuck with similarly low approval ratings a year in advance of the next election. Gerald Ford had a 44 percent approval rating a year before his loss to Carter. Johnson had a 41 percent approval rating in November 1967, and although he was eligible for another term, he opted not to run. His vice president, Hubert H. Humphrey, did, and he lost.

In fact, since 1944 (when approval ratings first became reliable), there have been five cases in which the incumbent party’s president had an approval rating below 49 percent a year ahead of the election — as Obama almost certainly will, unless he finds the cure for cancer after our issue goes to print — and each time the incumbent party lost.