By Matt Frei
BBC News, Washington
There is no point in measuring the cold in Chicago in numbers.
Eight degrees below zero Fahrenheit actually feels like minus 20 thanks to the arctic wind whistling off Lake Michigan.
And that number is of course meaningless to anyone dealing in centigrade.
Chicago has not seen cold like this in a decade
The "frostbite advisory" on the local TV station was more helpful: "Do not stay outside for longer than five minutes," it warned.
Five minutes is not very long, especially if you're trying to make television.
"After five minutes," the advisory explained helpfully, "frostbite begins to set in."
The first symptoms are skin turning pale and waxy. So what's new?
From the air, Chicago looks like the set for a climate-change disaster flick.
The river is frozen solid. The Lake is a patchwork of creaking, groaning, squealing ice.
On the slippery ground, the brave citizens of Chicago huddle against the elements, trailing thick plumes of vapour from their frozen nostrils.
Our hotel, the giant Hilton, is hosting a global computer-geek conference.
I met a man from Japan who had never been to America but who had not stepped outside the hotel once since arriving on Saturday.
On Monday, Chicago had its coldest day in 10 years.
But the weather wasn't what was getting the locals down.
Helmets of hope became a cruel joke after the Bears' defeat
The Chicago Bears had just been crushed in the Super Bowl by the Indianapolis Colts.
Their defeat took place, oddly enough, in the unseasonable monsoon rains of Miami. The fact that they had lost in warm wet weather seemed to add insult to injury.
"Tropical Depression", the Chicago Tribune moaned in a banner headline.
The front page was adorned by a kneeling player in tears, sucking two fingers like a baby. Had he done this in Chicago his fingers would presumably have flash-frozen to his lips.
The ornamental lions outside the magnificent Art Institute are wearing giant Chicago Bears helmets.
Before the Super Bowl the gimmick was funny.
If they had won, the helmets would surely never have been taken off.
But now they look like an old joke accumulating a thin layer of snow and ice.
Super Bowl to superstars
Our reason for being here was America's political Super Bowl, in which Chicago will play a central role.
The presidential election will take place in November 2008.
But the jockeying, the campaigning, the fundraising and the round-the-clock bloviating of media pundits, armchair strategists and kitchen-stool pollsters has already started.
The line-up of candidates on both sides looks like a remake of "Cheaper by the Dozen".
Mr Obama's political career began in Chicago
At the last count there were more than 12 hopefuls who had officially declared or set up that wonderful euphemism of campaign politics: the exploratory committee.
This allows candidates to raise money and hire staff, while still leaving a foot in the exit door.
Like the rest of the country Chicago is gripped by the face-off between the two superstars in the Democratic line-up.
Senator Barack Obama, the darling of the media - the man who turns heads in Hollywood and is beginning to empty corporate pockets on Wall Street - is a local boy of sorts.
Born to a mother from Kansas and a father from Nairobi, he grew up in Hawaii and Indonesia.
But he has spent his political life in Chicago, first as a law professor at the University of Chicago, then as a state senator and since 2004 as a US senator.
It is in Chicago that his wife Michelle and their two daughters live.
On Saturday he will officially announce his bid for the nomination in nearby Springfield, Illinois' state capital.
The senator has moved his campaign headquarters to his home turf in the Windy City from a city that specialises in hot air -Washington DC.
The mood among the growing army of staffers is the combination of giddy excitement and throat-drying fear produced when you're riding a tiger.
The challenge is to rake in the money and crank up the campaign machine without losing the Obama magic that has enthralled the rank and file of the Democratic Party and much of the media, raising expectations to alarming heights.
I watched Mr Obama in Washington last week at a working breakfast for all the Democratic hopefuls.
He was the only candidate who didn't bother with fanfare music or posters waved by his entourage.
His message was sombre and poignant and when the assembled crowd of hacks, politicians and campaign veterans wasn't clapping they were listening in hushed silence. That doesn't happen often.
Magic or machine?
Hillary Clinton lacks some of Mr Obama's magic.
For one thing, she has been around a lot longer.
Her campaign is not a blushing debut but a well-thought makeover. Her image has warmed, her edges have softened, her voice is more honey than ice and she has years of experience.
Hillary Clinton is trying to soften her image
Barack Obama has the magic. Hillary Clinton has the machine.
In the places that choose their presidential candidates early, like New Hampshire, Iowa and South Carolina, her "ground war" is in full swing.
As the weather freezes, her image seems to be thawing.
In the Park Ridge suburb of Chicago where she grew up, voters are divided.
At the Pickwick Restaurant, where they even serve a Hillary burger - with olives and capers to add some bite, I suggested flippantly - Helen the waitress was torn between Mr Obama and Mrs Clinton.
"I love them both. They're both different and new," she said. "Let's wait and hear what they have to say in the campaign."
"At least we are all paying attention," shouted George, the cook, from the kitchen. Indeed.
With a stomach full of Hillary burger - and mildly regretting the aftertaste of olives and capers - it was time to get wrapped up and venture outside.
Through the slit between scarf and woolly hat, I could just make out the headline in a discarded newspaper stuck to the pavement: "Obama to quit smoking."
As I scuttled towards my car several things occurred to me.
I didn't even know he smoked. Then: If he's worried that smoking is going to kill his campaign, then he's not the man I thought he was.
It was after all Mr Obama who once declared that he had smoked pot at university and "inhaled". "Wasn't that the point?" he added.
But if the senator is afraid that cigarettes will finish him off then he should definitely quit, in the full knowledge that Chicago will be supporting him all the way.
The Windy City may have lost the Super Bowl but it is lucky to have two of the most intriguing candidates in the most open presidential election since 1928.
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