Think the "battleground state" is a new development? The October 6, 1908 NYT reports:

New York, New Jersey, Ohio, Indiana - these are the determining States. In both National committees this is fully recognized. From now until election day these are the States in which both the Republican and Democratic managers will centre their efforts. Into these states will go both Mr. Taft and Mr. Bryan. Bryan will reserve New York for the last.

The story goes on to show that candidates have ceded states to their opponents for quite some time:

Norman E. Mack, Chairman of the Democratic National Committee, came here to-day and told the newspaper men that he left for the Republican in the East only Pennsylvania and two or three of the New England States, but some of his colleagues at the headquarters, when they learned that he had not put Pennsylvania into the debatable column, protested that he was altogether conservative.