Between now and February 5th Barack Obama and Hillary Clinton will fight 25 contests for the Democratic presidential nomination in an intense and gruelling process of political winnowing. Mr Obama's triumph in the Iowa caucuses on a turnout among Democrats nearly double that of 2004 gives him a great start. That is the major political fact to emerge from Iowa, especially his appeal for change to younger voters and women.
But he remains vulnerable in more typical states where Ms Clinton's organisation is stronger while he has yet to differentiate himself from her in terms of issues rather than personality - unlike John Edwards, who is more clearly on the populist left of the party.
Mike Huckabee's victory in the much less well-attended Republican caucuses confirms his appeal to the party's evangelical Christian base and the attraction of his unorthodox case for greater social protection. Both he and Mr Obama seek change that upsets both their party establishments. In New Hampshire on Tuesday next, Mr Huckabee will not have the same salience and still has a long way to go for the nomination. But his easy defeat of Mitt Romney puts that much more orthodox figure decidedly on the defensive against potentially stronger contenders like Rudy Giuliani and John McCain.
The increased compression of campaigning in the first two months of 2008 has given Iowa and New Hampshire greater prominence as agenda-setters. Their contrasting political cultures provide a fascinating glimpse of different sides of what preoccupies US public opinion. Several distinctive themes have emerged from these encounters. The economy, healthcare, family values and the Iraq war were the dominant issues for Iowa voters, revealing a new uncertainty about whether politics can deliver benefits for ordinary people and a conviction that they ought to. This feeling was summed up in Mr Huckabee's remark that the result shows "people are more important than the purse". New Hampshire will test that perception in a more individualistic state.
Some wider dynamics on the 2008 campaign have also been revealed in Iowa. The great increase in Democratic turnout shows how much the dislike of President Bush has mobilised the party's supporters. Observers must remember that Independents are nearly as numerous as Republicans and Democrats. The fact that they have flocked to Mr Obama rather than Ms Clinton is telling if it continues. Her carefully cultivated image of inevitable nomination and inherent electability have been dealt a blow and must be quickly retrieved if she is not to lose momentum rapidly and pitilessly.
For all that Mr Obama's message of change so far lacks real issue-related substance it undoubtedly appeals to a real sense of frustration among the electorate. If he can maintain that momentum he will become an unstoppable force. His appeal as a black American to white voters and his ability to upstage the formidable Clinton team on this occasion look like enduring political facts.