Among the nine key swing states, Pennsylvania is the closest thing to a must-win for President Obama. Until the first presidential debate, he was comfortably ahead in the Keystone State. But two polls taken either entirely or partly after the debate show Obama’s lead having dwindled to just 3 percentage points or less.

According to Real Clear Politics, the last five polls taken in Pennsylvania before the debate showed Obama leading Mitt Romney by an average of a little over 8 points. Now, a Siena poll taken partly before and partly after the debate shows Obama up by 3 points (43 to 40 percent), with a large number of undecided voters still up for grabs. A Susquehanna poll taken entirely after the debate shows Obama up by 2 points (47 to 45 percent) and notes that Romney’s net approval rating is now up substantially — from an average of minus-4 points (39 percent favorable, 43 percent unfavorable) in that poll from June through September, to +6 points (48 percent favorable, 42 percent unfavorable) in the latest poll. That’s higher than Obama’s net approval rating of +3 points (50 percent favorable, 47 percent unfavorable).

According to RCP, Obama is currently running almost 8 points behind where he was versus John McCain in Pennsylvania polling at this point in 2008. Obama went on to beat McCain in that state by just over 10 points, which suggests that his advantage this time around could be in the range of just 2 or 3 points. No Democrat since Harry Truman has won a presidential election without winning Pennsylvania.

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