PITTSBURGH - This is the political contest that wasn't supposed to happen in the place that wasn't supposed to be important at a time that wasn't supposed to be contentious.

A little while ago, Michelle Obama treated her entourage to seven cheeseburgers for $63.66 in Pittsburgh's most colourful (and, to my taste, most flavourful) bar and grill. Across town, Hillary Rodham Clinton appeared on the city's South Side and proposed $7-billion in research tax incentives to encourage growth in high-tech and environmental sectors of the economy.

None of this was supposed to occur. The struggle for the Democratic presidential nomination was supposed to be settled two months ago, the candidates and their spouses weren't supposed to come to Pittsburgh, and the Pennsylvania primary was supposed to count as much as it always does. For nothing.

Not this year, not with the Democrats, not with Barack Obama and Hillary Rodham Clinton. They're still slugging it out, slogging through Pennsylvania - from the Great Lakes to the Atlantic, through mill towns, mining communities, university centres, wrecked monuments to early 20th-century industries, shiny new high-tech and hospital complexes representing 21st-century hopes - hunting for convention delegates, manoeuvring for advantage, sliming their rivals. The soldiers who mobilized in August, 1914, for the First World War thought they'd be home for Christmas. The activists, media advisers, pollsters and consultants who mobilized for the nomination fights in January, 2008, thought they'd be home before spring.

The equinox has come and gone, and still American politics have not reached any sense of equilibrium.

Thus Pennsylvania. Thus the Pennsylvania Primary. Thus burgers at Tessaro's pub and tech-talk at the International Brotherhood of Electrical Workers. And it isn't over yet. The balloting here isn't until Tuesday. We're exhausted.

The candidates have come to a state that isn't accustomed to being a staging area for presidential politics. Grim-faced farmers in Iowa and cranky merchants in New Hampshire are accustomed to grilling the candidates, bearing down on them with questions and demands and neatly printed manifestos, posing with them for pictures in the soybean fields and against the backdrop of the White Mountains. For a generation, Pennsylvanians have believed that presidential politics is something that is engaged in by other people.

The new battlefield in American politics is anchored by two cities with rabid sports fans, elegant symphony orchestras and grand philanthropic traditions. But as big as Philadelphia and Pittsburgh loom, they are very different and very far from each other - 300 miles apart, almost exactly the distance between two other very important and very different North American cities, Montreal and Toronto. The way to tell the difference - between Pittsburgh in the west and Philadelphia in the east, as between Montreal in Quebec and Toronto in Ontario - is to assert to a resident of either of them that the two cities are basically the same.

The best Baedeker guide to Pennsylvania may be the most frequently quoted description of the state, attributed to the Democratic political consultant James Carville, now a big wheel in the Clinton campaign: Pennsylvania is Pittsburgh and Philadelphia, with Alabama in between.

There is a lot of in-between in this massive state, and as much as the 2008 candidates are campaigning in Pittsburgh and Philadelphia - the truth is that we've got a little bit sick of them, and there are still three days to go - a lot of the campaigning has been going on in the vast in-between. In State College, home to Pennsylvania State University. In Scranton, the setting for the television comedy The Office. In Erie, on the lake, not far from the vineyards, spiritually in-between, if not technically so. In rural crossroads where the Amish buggies crawl along the streets. On dairy farms struggling with new economic forces. In small factory and coal towns, which have it so bad they may not even realize we're in a recession; they've been down so long it looks like up to them.

In short, we're talking Ontario without the "u" in colourful. Here's the calculus. Ms. Clinton began with a big lead, but, like all big leads in this political season, it has shrunk. She's behind nationally in convention delegates and in the popular vote, and needs a big win here - a very big win - to maintain the argument that her nomination is plausible. She has defied all expectations by winning the support of white males along with the women she expected all along to carry. Mr. Obama's power base consists of the educated, well-off voters and - curious bedfellows in these times - black voters. A party once ridiculed as a collection of women and blacks will nominate either a woman or a black for president.

Ms. Clinton's doggedness and grit have been on display here, along with Mr. Obama's rhetoric and grace. They are each half of the complete candidate for the 21st century, but Democratic voters have to choose between them. They can't have both. Ms. Clinton has to clean up in the centre of the state and in the collar around Philadelphia. Mr. Obama has to win big in centre-city Philadelphia and in Allegheny County, which is Pittsburgh and its near suburbs.

All this is happening in a state notably inhospitable to both blacks and women, with only one of each among the 21 representatives that Pennsylvania sends to the U.S. House of Representatives in Washington. No black or woman ever has served as governor of Pennsylvania - Massachusetts, no hotbed of tolerance, has had both a female and a black governor within the past five years - and only seven states have a lower percentage of women in the state legislature. Not long ago, in response to a question I asked him in a meeting at the Pittsburgh Post-Gazette, the Democratic Governor, Edward G. Rendell, created a flurry when he said that his margin of victory in his last election was enhanced because his opponent, the former Pittsburgh Steelers star Lynn Swan, was black. Some things not said elsewhere are said here.

Pennsylvania is an old-fashioned state, with established habits and sturdy traditions, which is why all of the movement in this election is so deeply disorienting. About 300,000 new voters have been added to the rolls - more than four-fifths of them Democrats. That figure includes hordes of already registered Republicans who have re-enrolled as Democrats.

Many of the new voters enlisted so as to support Mr. Obama, who, like the civil-rights activist Jesse Jackson and a series of leaders of the religious conservative movement, has helped to reinvigorate American electoral politics in the last generation. But some of the Republicans in exile - it is utterly impossible to estimate how many - became Democrats just so they would be able to vote for Ms. Clinton, the theory being that they would love to mount a campaign against the former first lady and stoke the old memories of infidelity and impeachment. The movement of anti-Clinton Republicans for Ms. Clinton is known as Operation Chaos, aptly.

But overall, Pennsylvania, commonly regarded as a "swing state" and thus favoured by the attention of candidates from both parties in the autumn of presidential-election years, seems to be moving slightly into the Democratic column. The Republicans have lost this state in the past four consecutive presidential elections, though Senator John F. Kerry's margin of victory over President George W. Bush four years ago was only 2.5 percentage points - a mere 144,248 votes out of 5,769,590 cast across the state. All those new Democratic voters may make it even tougher for Senator John S. McCain of Arizona, the presumptive Republican nominee, to prevail in November here. Right now he's barely visible in Pennsylvania.

For more than two centuries, Pennsylvania has been known as the Keystone State, probably because, geographically if not culturally or politically, it plays the role of the keystone that holds all the other stones of an arch in place. But for all that, Pennsylvania almost always has been ignored in presidential-nomination politics. Most election years, its April primary is too late to be of any consequence, and over the decades the state has developed a stunted political culture, contributing not one first-rank figure to American civic life since Benjamin Franklin, and he died less than a year after the French Revolution. (Indeed, Pennsylvania's only president, James Buchanan, who preceded Abraham Lincoln, is remembered dimly, and then only for helping to bring on the Civil War. His era, 1857-1861, is regarded as a peculiarly dark and rancid period, and he is almost universally considered the worst chief executive in American history.)

Now, there are Clinton and Obama signs on nearly every street, advertisements on every television station, stories on every front page, proclamations of a new era on every set of lips. For the first time since the Battle of Gettysburg, July 1-3, 1863, Pennsylvania truly is the epicentre of American life. We're a little self-conscious about all this, to be sure, but let me whisper the truth: We love the attention, and for a few days more, Barack Obama and Hillary Rodham Clinton love us. They really, really love us.

By David Shribman, Globe and Mail, April 19, 2008