The high gas prices are one tangible way that the public feels the bite of a weak economy and those gas prices are doing no favors for McCain. Using data from the American Automobile Association and state-by-state polling data available on Pollster.com*, I examined the relationship between the average price of a gallon of regular gasoline and the polling margin between Obama and McCain in each state. You can see how the states plot on these measures in the figure below.
Notably, there is a significant increase in how Obama fares against McCain as gas prices in a state increase. The regression line indicates that for each 10 cent increase in the cost of a gallon of gasoline in a state, Obama's margin against McCain improves by approximately 6.7%. In other words, Obama does well where people pay more to fill up their cars while McCain fares better where gas prices are lower.
Of course, the relationship is far from perfect as there are several outliers. Vermont gives Obama his biggest advantage over McCain despite the fact that the state pays slightly below the national average for gas. On the other hand, Alaska has some of the most expensive gas in the country, but also tilts strongly in McCain's favor.
Nonetheless, a struggling economy can effect people in many ways and one of the most obvious is at the pump. Filling the tank may serve as a frequent reminder about the economic state, something that doesn't help any Republican candidates, including McCain, in this election.
*When available, I used the pollster.com averages for each state. When there were not enough data points to generate an average, I used the most recent available poll.