The new NBC/WSJ poll showing McCain significantly closing the gap with Obama also finds:

Yet perhaps the biggest factor keeping the presidential race close has been Obama's inability to close the deal with some of Hillary Clinton's supporters. According to the poll, 52 percent of them say they will vote for Obama, but 21 percent are backing McCain, with an additional 27 percent who are undecided or want to vote for someone else.

What's more, those who backed Clinton in the primaries -- but aren't supporting Obama right now -- tend to view McCain in a better light than Obama and have more confidence in McCain's ability to be commander-in-chief.

Part of Obama's job since winning the nomination in June was to shore up his differences with Clinton supporters. Most pundits -- including this one -- believed that was just going to be a matter of time. Maybe it still is. But this latest poll suggest a couple things: 1.) McCain has done a much better job than Obama on solidifying his base; 2.) Obama's decline in recent weeks has as much to do with McCain attacks as it does with his own inability to bring Clinton supporters on board.

Which points to one obvious question: Does Obama need Clinton?