By Kevin Connolly
BBC News, Columbia, South Carolina
The campaigning machines which propel America's presidential hopefuls through the primary season are curious movements, forever looking forward even in moments when the rest of us might be tempted to contemplate defeat or savour victory.
Consider the case of Mitt Romney, for example, the Republican who has spent a fortune already in this race, and has only mixed fortunes to show for it.
Mr Romney has had a good weekend, if not quite a great one.
Knowing that he had no real chance of victory in South Carolina he was the only Republican candidate to campaign seriously in the Western desert state of Nevada and the tactic paid off. Mr Romney won handsomely.
True, John McCain won the larger prize on offer in their party this weekend, taking first place in the primary in South Carolina.
That is important because everyone in American politics knows that every year since 1980, whichever Republican candidate has won here has gone on to win the party's nomination - and more often than not the presidency.
John McCain knows it better than most - he lost here in 2000 to George W Bush and never recovered, so victory this time around was particularly sweet and particularly significant
By the time Senator McCain had taken the stage in Charleston to tell his supporters "it's been a long time, but what's eight years between friends", Mr Romney had already popped up on television live by satellite from Florida, which stages the next bruising encounter between the Republican front-runners on 29 January.
The trick is to somehow keep looking forward even in the darkest moments.
When Mike Huckabee of Arkansas took the stage to acknowledge that he had lost in South Carolina, he told his supporters that they should be asking "What now?", rather than "What if?", as they contemplated the challenges to come.
Still, defeat in South Carolina was a real blow for Mike Huckabee.
He is personable and witty, and as an ordained Baptist minister he can rely on the support of the conservative Christians who represent a powerful force within modern Republicanism.
But he is a southerner too, linked by accent, culture and instinct to his fellow-southerners in South Carolina.
He needed to win here because his background and simple brand of Bible-believing Christianity is going to be a tough sell in places like California and New York.
A South Carolina win would have been a boost for Mr Huckabee
If it's true that winning in South Carolina is statistically a sign that a Republican candidate will go on to win the nomination, it's even more firmly held that winning two out of three contests in Iowa, New Hampshire and South Carolina seals the outcome of the race.
John McCain has now done exactly that, but this year's Republican race is so tight, and the field so crowded, that that statistical predictor may not be quite as reliable as usual.
Still, it is clear the balance of advantage now lies with Mr McCain, the oldest candidate in the field at 71.
Next comes Florida, and at last we will be able to stop making the point every time there is a Republican primary that Rudy Giuliani has been keeping his powder dry through the early campaigning.
Florida is the place he has been keeping his powder dry for. He has been campaigning there all along, although he has not been attracting much publicity in the process.
If he wins the Sunshine State we will hail Mr Giuliani as a tactical genius who knew better than to waste his energies toiling through the snows of Iowa and New Hampshire.
If he loses badly, we will remember him as having made one of the great campaigning errors of all time. We will see.
Mrs Clinton called her Nevada win a 'huge victory'
On the Democrat side, after lively caucuses in Nevada, not much has changed.
There is tremendous energy on that side of American politics, as demonstrated by the high turn-out, and once again, after a tight contest between Hillary Clinton and Barack Obama, it was advantage Senator Clinton.
There were a few interesting straws in the wind - Hillary did well among Hispanic voters for example - but the Democrat contest in South Carolina next weekend will produce another tough battle, with little to choose between the front-runners.
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The Palmetto State, as it is known, has a significant voting population of African-Americans and we will get some indication there of how that community is viewing Barack Obama's campaign to become the first black president of the United States.
America will most likely vote for a Democrat in the upcoming election
Kevin Bierfeldt, New York
A victory for him there would mean the momentum on the way into Super Tuesday, the day in February when 22 states vote, would be evenly divided between the two leading Democrats.
Again though, look out for the ways in which both are continually looking forward.
Before the vote in Nevada, Barack Obama took time to go fund-raising in California and shortly after it Hillary Clinton was already miles away in St Louis, Missouri, her tired voice showing the physical strain of campaigning.
The pace of campaigning, already extraordinarily intense, is likely to intensify still further as Super Tuesday looms.
Whoever wins is going to need stamina, as well as strategy.