I think the math for Silver’s (and my) conjecture requires only that Romney’s popular vote margins in the states he wins exceed the popular vote margins for Obama in the states he wins. I can imagine Obama squeaking out victories in Ohio, Colorado, and Wisconsin, for example, while Romney wins big in the South and most of the Midwest.
Here is an example of why the electoral college favors the GOP, from 2008:
Wyoming - McCain won by 80k votes...and he got 3 electoral votes.
Alaska - he won by 70k...picked up 3
Montana - won by 10k...picked up 3
North Dakota - won by 28k and got 3
South Dakota - 35k margin picks up 3
Taken all together, the votes per electoral vote is 45,000.
Now look at California for Obama. 55 electoral votes, with a margin of 3.2 million.
The number of votes necessary to get each of those 55 electoral votes: 58,000...or 28% more than it took McCain.
This phenomenon is not because the races in the 3 EV states were close. Some weren’t. Its just because there aren’t many people in the 3 EV states.
Obama can expect to get no 3 EV states. So he would never benefit from this phenomenon.
This is why it would be very difficult for Romney to win popular and lose EV. And just consider who the biggest proponents of scrapping the electoral college are...they’re democrats. That’s because they have figured out that it hurts them.
I would not worry at all about Nate Silver’s scenario.