His lead has shrunk considerably since Newsweek conducted their last poll - Obama has dropped 11 points versus McCain in under a month.
Obama now leads 44 to 41, just within the margin of error. The poll slightly oversamples Democrats to Republicans. However, this was a poll of registered voters, not likely voters. Democratic registered voters typically show up in smaller numbers than Republican registered voters, so this probably erodes Obama's support still further. The poll also oversampled women, who are even less likely to actually go out and vote, and who support Obama in greater numbers to begin with.
Considering that at this point in the election cycle the Democrat candidate is usually way ahead of the Republican candidate (Kerry was ahead by about 9 points in 2004, Dukakis by 16 in 1988. Both lost the election) the Obama campaign should be entering panic mode right about now.
How did this happen? Obama proved that he stands for absolutely nothing. His major drop in the polls has come just after he has tried to rebrand himself as a centrist. All he really did was prove that his views are as changeable as the wind, and that all that hope and change stuff means nothing. All of his flipflops on Iraq, campaign financing, FISA, the death penalty and so on show that he is just another politician who will do and say anything for a vote. It completely undercuts his image as an outsider purist untainted by business-as-usual politics.