From the New York Times:

In a striking last-minute shift, the Romney campaign has decided to invest its most precious resource — the candidate’s time — in a serious play to win Pennsylvania.

Mr. Romney’s appearance here on Sunday could be a crafty political move to seriously undercut President Obama, or it could be a sign of desperation. Either way, his visit represents the biggest jolt yet in a state that was until recently largely ignored in the race for the White House.

Over the last several days, with polls showing Mr. Obama’s edge in the state narrowing, Republicans have sprung into action and forced the Democrats to spend resources here that could have gone toward more competitive battleground states.

In a previous post, I discussed why it might be profitable to move ad budgets to non-battleground states if battlegrounds are saturated with coverage. Basically, if you are strong in the battleground states, by spending money in a non-battleground state, you can divert your competitor’s resources there too and, if you are weak, you have to go there out of necessity. Hence, you cannot determine whether Romney has momentum or not from his ad spending strategy. But what about his travel strategy? A candidate’s time is scarce, unlike his billion dollar ad budget, so it must be rationed carefully. So, what can we infer from the fact that Romney is campaigning in PA on Sunday?

The New York Times Electoral Map is useful to think through various scenarios. Romney needs to win NC, FL and VA to have any chance of making it to the White House. Visiting and campaigning in those states is part of his defensive strategy – he has no option but to devote attention to them. As everyone is saying, OH is the key to the door of the White House.

What should Romney do if he knows the President has a significant lead in OH? There is no point campaigning even more in VA, NC or FL because even if he wins them, he cannot win the Presidency without some other states. He has to shift campaigning to some other states to make it to 269 electoral votes so the House can give him the Presidency. Then, it might seem he should shift his time to CO, IA, NH or WI. But to make it to 269, Romney would have to win WI, CO and one of the other two (according to my manipulation of the NYT map). This is a tall order. On the other hand, if he wins PA with one of these other four states, he is through. MN does not have enough electoral votes to counterbalance a loss of OH so the choice comes down to MI versus PA. If Romney is closer in PA than MI, that breaks the tie. So, to summarize, it makes more sense to campaign in PA and try to win one other state than to try to win WI, CO and one other.

What if Romney is ahead in OH? Then, he still needs one of CO, NH, IA or WI to win. Not focussing on those states and defending OH, VA etc. exclusively could cost him the Presidency. In this scenario, Romney does not have the luxury of the time to campaign in PA.

Hence, the Romney campaign’s focus on PA only really makes sense if they know President Obama is well ahead in OH. There is no Romentum.

UPDATE (11/3):  From NBC,

On party ID
In these surveys, Democrats enjoy a nine-point party-identification advantage in Ohio and a two-point edge in Florida. Republicans have argued that a nine-point advantage is too large in this current political environment; it was eight points in the Buckeye State during Obama’s decisive 2008 victory.

If you cut that party ID advantage in half, Obama’s six-point lead in Ohio is reduced to three points.