US Election: All Eyes on Ohio and Virginia

Three weeks ago, I gave a list of 10 states totaling 125 electoral votes that will likely determine the outcome of the 2012 U.S. presidential election - both candidates need a majority of those 125 votes to win. At the time, Obama led in all 10 of those states, but of course, Romney has gained substantial momentum since then with his strong performance in the first debate.

Right now, based on state-by-state polls, I'd classify those states as follows:
- Clear Obama lead: PA (20), MN (10)
- Obama edge: OH (18), WI (10), IA (6), NV (6)
- Tossup: VA (13), CO (9), NH (4)
- Romney edge: FL (29)
This gives a 277-235 lead for Obama in the electoral college.

(By the way, some other states like MI (16) on the Obama side or NC (15) and AZ (11) on the Romney side are also relatively tight, but they are very unlikely to be decisive. In the above list, it looks like MN is also firmly in the Obama column.)

However, as Nate Silver has pointed out time and time again on his blog, the national polls have lately been somewhat more favourable to Romney than the state polls. If we give the "Tossup" states to Romney, we end up with a very tight race: 277-261. This is very similar to Mr. Silver's "now-cast", which gives Obama a 280.4-257.6 lead (he has NH as leaning Obama).

We can now probably narrow down the list of states to watch to the seven "Obama edge" or "Tossup" states above: OH, VA, WI, CO, IA, NV and NH. This doesn't mean that PA will definitely go for Obama or that FL will go for Romney, but if those things don't happen, the winner is likely to have over 300 electoral votes.

Ohio and Virginia are the keys to the election: if a candidate wins both of these, he forces his opponent to run the table on WI, CO, IA, NV and NH.

If Ohio and Virginia split, then, in most cases, whoever can carry three of WI, CO, IA, NV and NH wins. The exceptions are:
- OH, WI and any one of CO/IA/NV is enough (so VA, NH and two of CO/IA/NV is insufficient)
- OH, CO and either IA or NV is enough for Obama (so VA, WI, NH and either IA or NV is insufficient for Romney)

Polls close at 7 in Virginia, 7:30 in Ohio, 8 in New Hampshire, 9 in Colorado and Wisconsin, and 10 in Iowa and Nevada (all times Eastern) - conveniently, the two most important states close first.