What follows will be old hat to most readers, who’ve been wargaming paths to 270 for six months now, but I’m thinking it might be useful to casual readers who are stopping by tonight and tomorrow because their interest in the election is peaking. Simple question: Which states does Romney need to win to clinch the presidency? BuzzFeed tried to answer this earlier today with a flow chart, but it doesn’t give you any sense of whether individual battlegrounds are likely right now to break red or blue. So here’s how I’m approaching it. Right off, to simplify things, I’m assuming Romney wins North Carolina (15 EVs) and Obama wins Nevada (6). Neither one is a lock but they seem to be the surest bets among swing states. Needless to say, if you live in either of those states (or any other state), you should hustle on down to the polls tomorrow and vote anyway. An upset for O in NC would all but guarantee that he wins the election, and low GOP turnout in Nevada would imperil Dean Heller, whom the party desperately needs to win to have a shot at retaking the Senate. No excuses. Vote, vote, vote.

If you assume NC and NV break red and blue, respectively, then the election starts with Obama at 207 EVs and Romney at 206, with seven states effectively left to decide things. Which brings us to…

The prerequisites: Florida (29), Virginia (13)

Romney leads by 1.5 points in the RCP average in Florida, his best showing in any battleground state. He’s led there for weeks and is widely expected to take the state. He’d better: 29 EVs would be next to impossible to replace. Virginia is more tenuous, with Obama actually holding a very slight lead in the poll of polls right now. Romney could replace those 13 EVs by winning one or more states listed below, but he’s led in multiple polls in Virginia over the past month and seems to be favored there by most analysts. If he loses a squeaker to O, there’d be little margin for error with the remaining five states and it’d likely augur a bad, bad trend for the evening. The good news is that Obama is off his 2008 pace in early voting and Romney aides feel confident that the combo of coal interests plus military voters will nudge him over the line.

If Romney wins both prerequisites, he’s at 248 and within striking distance of the White House. He then needs 22 electoral votes from any combination of these five:

The deciders: Ohio (18), Wisconsin (10), Colorado (9), Iowa (6), New Hampshire (4)

Two obvious possibilities here.

Path One: Ohio + any other state. Even little New Hampshire would be enough to hand Romney the presidency if he locks down the Buckeye state and nothing else. (270-268!) The bad news is that Romney hasn’t led in a state poll in NH for nearly two weeks. The good news is that there’s no early voting there, so if you expect a nationwide trend of Republicans swamping Democrats at the polls tomorrow, then things look promising. In Ohio, Romney hasn’t led in any state poll since October 10 with the lone exception of Rasmussen, which had him up two points last week. Democratic early voting appears to be down, though, and Republicans traditionally outperform their national numbers slightly in Ohio. Tomorrow will be the ultimate test of Obama’s GOTV machine: Ohio Republicans know that the election will likely turn on their turnout, so it’s up to Team O to somehow blunt their numbers by dragging just enough half-hearted, disillusioned Hopenchange fans to the polls. Tall order.

Path Two: Wisconsin + Colorado + any other state. This is trickier, obviously, not only because it involves winning more states but because Romney actually trails by a wider margin in the Wisconsin RCP average than in Ohio. Colorado is within two points, though, and the GOP leads in early voting there(!). If CO comes through and Ryan/Walker magic leads to an upset in WI, then Romney can ignore Ohio and hope for Iowa to come through and win him the election. He trails there by less than 2.5 points and three different polls taken over the last two weeks or so have had him ahead by a point. If a red wave breaks tomorrow, it’ll probably carry Iowa with it.

So, what happens if Romney locks up the prerequisites in Florida and Virginia and then wins Colorado, say — but ends up losing narrowly in both Ohio and Wisconsin? Now he’s stuck at 257 and not even winning both Iowa and New Hampshire will get him to 270. Either he needs a huge upset in Nevada, which is unlikely if OH and WI are trending blue, or he needs…

The longshots: Pennsylvania (20), Michigan (16), Minnesota (10).

Actually, neither PA nor MI is a true longshot. Romney’s closer to Obama in the RCP average for each state (3.8 points in both Pennsylvania and Michigan) than he is in Wisconsin (4.2 points). He trails a bit more distantly in Minnesota (5.2 points), but even there, some polls have him either slightly ahead or within three. I’m listing these states here because they’re reliably blue in presidential elections and because the GOP has spent less time and money contesting them than it has in, say, Wisconsin. But if Romney runs into problems in the “decider” states, or if Virginia somehow falls through and he needs to find 13 EVs somewhere, obviously these will be crucially important. My hunch, though, is that if he’s losing narrowly in the more competitive midwestern states, like Ohio, then it’s unlikely he’ll reverse that trend in the less competitive ones. If any of the “longshot” states are turning red, it’s probably because there’s a huge Republican wave and Romney’s cruising to a landslide win. Here’s hoping.

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If all of the above is too complicated, here’s a much simpler way to understand Romney’s task. Assuming Obama wins Nevada, all he has to do to win the election is take the big four in the Rust Belt and midwest — i.e., Ohio, Wisconsin, Michigan, and Pennsylvania. That would put him at 271. Romney must win at least one of those four states to have any chance of victory. If he doesn’t, then he’d have to win every other battleground state — Nevada included — or else.

Via Christian Heinze of GOP12, here’s the Rove map.