And it wasn't in 2008 either. A Presidential swing state is one that if it swings to either candidate then that candidate has a better chance of winning the Presidency. The state with the smallest margin of victory that would put the candidate over the top is the one that swung to him, giving him the Presidency.
Ohio would've put either candidate over the top. Hence the "O."
The states with the smallest margin of victory to get George W. Bush to 270 electoral votes were Florida, Colorado, Nevada, and Ohio. Bush also won New Mexico and Iowa but didn't need to win either to get to 270. Had John Kerry won Iowa, New Mexico, and Ohio, the three states where he was closest, he would've won the Presidency.
All of the states above voted for Barack Obama. Barack Obama also won North Carolina, Florida, and Indiana. Had John McCain gotten enough votes to win those states he wouldn't have won. He would've also needed to win Ohio, Virginia, Colorado, and Iowa to capture the Presidency. If I'd told you McCain had scored enough votes to win Indiana you wouldn't have known whether he'd have won. If I told you he won Minnesota and Pennsylvania you would've known he would've won.
We know no one is going to advertise in Texas or California. Those are in the bag for each party. If either party lost these states, no matter how much they were outspent, they'd have lost in a landslide. So no one advertises there.
Thus, there's not a great reason for Republicans to advertise in Florida. If they (Republicans) don't do well enough in Florida, it's over. Thus, there's not a great reason for Republicans to advertise in Florida. If they do well enough nationally, the Republicans will win Florida. The Democrats also don't need to waste their resources on Florida. If they're able to take Florida, they'll have likely taken enough states on this list to put them over the top. They could win Florida. There's enough variance that states move up and down the list, but it's unlikely the Democrats won't have won enough states on this list to get to 270 if they win Florida. Since eeking out a victory isn't necessary, then why waste your resources?