They’re off. The sprint to the general election has begun, and so have the calculations, interrogations, interpretations, and exaggerations. The last two elections were nearly dead heats. This one begins the September stretch with John McCain and Barack Obama very close in the polls — and with more than half the states already ruled out of contention by one side or the other.

Right now Senator McCain is enjoying an unusually large convention bump — the second biggest in modern history, as measured by the Gallup Poll. Ordinarily candidates roaring out of their nominating conventions get a boost of 5 or 6 percentage points, with Democrats slightly on the high end (6.2 points) and Republicans slightly on the low end (5.3 points).

The McCain swing of about 10 points was surpassed only by Bill Clinton’s 16-point bump in 1992, but remember: Jimmy Carter had almost the identical bump in 1980 (10 points) and lost the election decisively. Only half of the candidates who scored the eight biggest bumps in modern history won in November.

In some ways this election is wide open: no incumbent seeking another term, as in 2004, 1996, 1992, 1984, 1980; no vice president seeking to move up (2000, 1988); no former vice president trying to claim the big prize (1984); no senior party regular running with the conviction that it’s his turn (1996).

But in some ways this campaign of big implications — first black presidential nominee, perhaps the last chance for a Vietnam veteran to claim the presidency, a challenge to party orthodoxies no matter who prevails — is being played on a remarkably narrow field. Here are its boundaries:

Intensity. For months it has been clear that the Democrats felt more urgency about the election, approached it more passionately, and were positioning themselves with more unity than the Republicans, who generally were underwhelmed by their choice of contenders.

No more. The Democrats still are fired with determination after eight years of President Bush, but the selection of Sarah Palin as Mr. McCain’s running mate has transformed Republicans’ outlook in two important ways: It has given them a new figure of inspiration and enthusiasm, and it has quieted their worries about whether religious conservatives would back the Republican ticket. No one wonders that now. Which leads us to:

Vice presidential politics. Commentators always say the identity of the vice presidential nominees matter and they’re always wrong. The most dramatic contrast between running mates in modern history came in 1988, when Senator Quayle, the Indiana Republican, opposed Senator Bentsen, the Texas Democrat. Their showdown produced the only memorable line in the half-dozen or so such encounters in history — Bentsen to Mr. Quayle: “Senator, you’re no Jack Kennedy.”

This year may be different. Already the St. Louis showdown between Mrs. Palin and Senator Biden on October 2 is being billed as a political version of the 1975 confrontation between Muhammad Ali and Joe Frazier. Mrs. Palin’s utility to the Republicans may be promoting Republican turnout, not a small thing.

But Mrs. Palin still has much to prove, and this is according to the public, not according to the press: The latest CNN/Opinion Research Corp. survey shows that 50% believe she is qualified to serve as president. Not a bad figure — unless you compare it to the 70% who consider Mr. Biden qualified.

Swing states. No mystery here. Start with Ohio and Pennsylvania, add Colorado, Iowa, Michigan, Minnesota, New Hampshire, New Mexico. Maybe add Missouri, Nevada, Florida, Virginia, and one or two others. The election is going to be fought there. But forget about California, New York, and Texas. We already know who’s going to win those.

A lot of places that Mr. Obama took in the primaries and caucuses (Idaho, Utah) are no more likely to vote for him in November than they are for Vladimir Putin. The reverse is true for Mr. McCain in places like Vermont and Rhode Island. Though the Democrats see some opportunity in Indiana, where the polls are closer than the Republicans would like, in the end that state is a long shot for Mr. Obama.

Swing voters. The lesson here is that swing states and swing voters are not the same. Look instead for how swing voters behave in swing states, and realize that each state has a different set of swing voters. Catholics are swing voters, but how they swing doesn’t matter in New York. How they swing does matter in Iowa, where there are large clusters of Catholics along the Missouri and Mississippi Rivers. One more thing to remember: There are fewer swing voters this week than there were two weeks ago; almost a third were up for grabs from early July to Labor Day, but only about a fifth are undecided now.

Sex wars. There are strong indications that a big gender gap persists in American politics. Women have voted Democratic the last four elections in a row; men have leaned Republican in six of the past seven elections. George W. Bush’s margin among male voters was stunningly consistent: 11 percentage points in both of the last two elections. If Mr. McCain can hold the male edge in 2008, he will have an important advantage. And already, according to the latest Washington Post/ABC News poll, there has been a large swing of white women into the McCain camp since Mrs. Palin joined the ticket (20 percentage points).

Here’s an intriguing fact at mid-September of an election year, though: A higher rate of males approve of the Republicans’ female nominee for vice president than do females. Indeed, 62% of men have a favorable impression of Mrs. Palin, as opposed to 53% of women. A majority of women believe Mrs. Palin is not qualified to serve as president. Men feel strongly the other way.

It’s hard to draw many conclusions about the 2008 race at this distance from Election Day. But it is hard to resist the notion that the selection of Mrs. Palin changed the fundamentals of the race in several critical categories — and that an important factor, too early to measure, may be the reaction to her, or against her, in the weeks ahead.

© 2008 The Pittsburgh Post-Gazette