When this remarkable political year began, many Democrats were expecting a smooth passage to an historic nomination and a relatively easy presidential victory. That is hardly the case today.
Sen. Hillary Clinton was clearly the established favorite among a large field of challengers, blessed with far more financial and organizational resources than anyone else and the best brand name in Democratic politics.
She was the center of attention, not only for the Democrats but for the Republican candidates as well - the person they expected to face in the general election. As Republican aspirants were struggling to escape the downdraft of the self-immolation that had overtaken President Bush and the GOP Congress, with no assurance that anyone in the group could reassemble the scattered pieces of the Reagan legacy, all of them were focused on Clinton as the final barrier to keeping the White House.
Because the odds seemed so favorable for a Democratic victory in November, eight or nine candidates - with varying degrees of plausibility - decided it was worth the gamble to try to wrest the nomination from Clinton. The conventional wisdom at the start was that someone would emerge to challenge her after the first round of primaries and that she would probably defeat that unnamed opponent.
To everyone's surprise, the least credentialed of her opponents, young Sen. Barack Obama, turned out to have the personal and political skills that rocketed him past all the others. He beat the field in Iowa, stumbled briefly in New Hampshire and Nevada, recovered in South Carolina and emerged from Super Tuesday almost even with Clinton in delegates and ahead in the race for campaign dollars.
As the next phase of state-by-state contests begins, no one can claim the favorite's role in a Democratic contest that could go all the way to the national convention.
Meantime, on the Republican side, John McCain has resurrected his candidacy with a series of primary victories from New Hampshire through California, amassing enough delegates that his nomination is assured.
With Mitt Romney's withdrawal, and only Mike Huckabee, a friendly sparring partner, and the eccentric Ron Paul still running, Republicans can begin to focus on November. Their challenge is still difficult. The war in Iraq remains a heavy burden, its costs outweighing its dividends. The economy has turned down. And public weariness with the White House fuels a desire for change.
Nonetheless, McCain now has the luxury of time in which to mend his differences with some of his fellow conservatives and to pursue the independents whose support would make him a formidable contender.
Where Clinton was the measuring stick for all others in both parties during the past year, McCain now becomes the standard of comparison. As the Democratic race continues, the key question becomes, "Who matches up best against John McCain?"
That will increasingly be a factor for Democratic voters, who find themselves being fragmented on gender, racial and generational lines even in the absence of any serious policy or philosophical differences between the candidates.
And it will be even more central to the deliberations of the almost 800 "superdelegates" - elected and party officials who may represent the balance of power at the convention.
Both Clinton and Obama are now framing their campaigns as a riposte to John McCain. Clinton argues that, given McCain's authority as a warrior and as a defense expert, her experience and toughness are essential for the Democrats to have a chance.
Obama counters with the claim that it is only by providing the sharpest of contrasts - a generational gap linked to a flat-out denial of the strategic centrality of Iraq - that the Democrats can confront McCain and hope to win.
As I have previously noted, Clinton and McCain come close to matching each other when voters are asked to compare their experience and their ability to bring needed change. But McCain has a huge lead on Obama when voters judge experience, and Obama has a large advantage when it comes to promoting change.
The Democratic contest is more than a battle of personalities. It represents two sharply contrasting strategies for victory in November. The choice is one Democrats never expected to face.
E-mail nationally syndicated columnist David Broder at email@example.com.