Conventional wisdom says Pennsylvania is Clinton Country - blue collar, white and Catholic. But last week, it seemed Barack Obama had developed some momentum with the key white working-class voters that both candidates have struggled to attract.
Then Obama stumbled spectacularly. Speaking privately to fundraisers in San Francisco, he said: 'You go into these small towns in Pennsylvania and, like a lot of small towns in the Midwest, the jobs have been gone now for 25 years and nothing's replaced them... it's not surprising that they get bitter, they cling to guns or religion or antipathy to people who aren't like them or anti-immigrant sentiment or anti-trade sentiment as a way to explain their frustrations.'
As I write, the pollsters and pundits here in the US are furiously arguing over how significant a mistake he actually made.
Potentially, it is very damaging. Obama's cool analysis of the growth in significance of 'wedge' issues has been scooped up by opponents on both sides to position the Senator as an elitist, out of touch with small-town life. Senators McCain and Clinton have been 'shocked' by the suggestion that anyone in the land of opportunity is 'bitter' or frustrated.
By suggesting that God and guns were not part of great American traditions of freedom and worship but the by-products of being forgotten and excluded, he has handed a loaded pistol to his current and possible future opponents.
In my view, Obama now has to do three things.
Firstly, convince voters in Pennsylvania and beyond that his remarks were ones of empathy and support, not the stuff of value judgements and criticism.
Secondly, explain that the point of those remarks was to underline his commitment not to let those people continue to be forgotten by government.
Finally, and more aggressively, he will need to tackle Senator Clinton head on about who is better placed to help them find prosperity and security again.
The Obama momentum has stalled in Pennsylvania. He will have to find a way to reignite it.
Not only to stay close in this state, but to ensure that this does not create new momentum for the Clintons in Indiana, North Carolina, West Virginia and Kentucky, where there are many voters of similar profile.
Nick DeLuca is chairman of Open Road.