If current survey trends continue, Obama will finish with less than 50 percent in the polls. Even discounting the Nader vote (some people never learn), the undecided voters could tip the race either way. How will they break?

Since there is no incumbent, they cannot automatically be assigned to the challenger; and since turnout is likely to be huge, the current undecided voters will probably make their way to the polls and cast their ballots.

But for whom?

At the beginning of this contest, Obama effectively made the case that the election was a referendum on Bush's performance in office. Painting a vote for McCain as a desire for "four more years of the same failed policies," he made the most of Bush's dismal approval rating. Had he been able to keep the focus on Bush, he would likely have inherited most of the undecided vote.

But as Obama surged into a more or less permanent lead in October, animated by the financial crisis, he has assumed many of the characteristics of an incumbent. Every voter asks himself one question before he or she casts a ballot: Do I want to vote for Obama? His uniqueness, charisma and assertive program have so dominated the dialogue that the election is now a referendum on Obama.

As Obama has oscillated, moving somewhat above or somewhat below 50 percent in all the October polls, his election likely hangs in the balance. If he falls short of 50 percent in these circumstances, a majority of the voters can be said to have rejected him. Likely a disproportionate number of the undecideds will vote for McCain.

But don't write Obama off. His candidacy strikes such enthusiasm among young and minority voters that there is still a chance that a massive turnout will deliver the race to the Democrats. None of the polling organizations has any experience with -- or model for -- so massive a turnout, especially among voters notorious for staying at home. But the primaries proved that these young and minority voters will not stay home this time, but will vote for Obama. The effect of this increased vote is hard to calculate, but it may be enough to offset the undecideds who will vote for McCain.

But the basic point, one week before Election Day, is that even if Obama clings to a four- or five-point lead over McCain in the polling, the election is not over. The question is not so much how large his lead is over the Republican, but whether or not he is topping 50 percent. As long as the polling leaves him below that mark, he is vulnerable and could well lose.

Clearly, in recent weeks, McCain has been able to cast Obama as a leftist. He has made the issue of income redistribution central to the campaign. With the aid of Joe the Plumber and the discovery of Obama's Chicago PBS interview, in which he lamented the absence of redistribution of wealth, McCain has made the proposition seem central to Obama's ideology. The unprecedented power the bailout has given government over the banking industry raises the real specter of socialism in America. The banks have, effectively, been nationalized. How will government use its power over them? This new reality, coupled with Obama's professed pursuit of "social and political justice" through "redistribution of the wealth," is enough to send a shiver down the spine of those who embrace the free market as the key to economic growth.

The audacity of Obama's injection of a social democratic concept borrowed from Western Europe into American politics is stunning. And almost half the voters seem to be buying it.