One of the hottest topics of debate in the political world at the moment is whether on Nov. 7 we could be looking at President Obama having won the electoral college and Mitt Romney having won the national popular vote.

Much of that chatter is the result of findings from the Gallup national tracking poll that has shown the South unifying strongly behind former Massachusetts governor Mitt Romney (or against President Obama) and a disconnect between the dead heat in national polling and the slight Obama edge in swing state data. And, as Ezra Klein noted here, there is a real case to be made that the consolidation of the south makes an electoral vote/popular vote split seem more likely than it has in recent elections.

But, the counter-argument is also powerful. Democrats have won the popular vote in four out of the last five presidential elections -- including the 2000 race in which Al Gore got 500,000 more votes than George W. Bush nationally but wound up losing the electoral vote by two votes. (Worth noting: The last time one party had such a long run of popular vote victories was when Republicans won the popular vote in five of the six presidential elections between 1968 and 1992.)

The reason for that Democratic dominance in popular vote is due in large part to the party's strength (and Republicans' massive weakness) in heavily populated urban areas like Los Angeles, Chicago, Philadelphia and New York City.

The map below makes that case in a picture better than we can in words. It's a three-dimensional look at the county-by-county results from the 2008 election; the larger the column, the bigger the vote margin between the two candidates. As you can see, almost all the largest margins -- by vote differential -- are in Obama's favor and are very unlikely to change in any meaningful way on Nov. 6. (You can fiddle with this very cool map in a more interactive way here.) 

 It is, of course, conceivable that even with those massive margins out of a handful of heavily populated regions that Obama could still come up just short in the popular vote while winning the electoral vote -- although, if he does, he will be the first sitting incumbent to ever do so and be re-elected.  

But, it will take a lot of larger-than-expected Romney victories in heavily Republican counties and narrower-than-expected Obama magins in heavily Democratic counties for that to happen.

Possible? Absolutely. Probable? Probably not.