1960s/70s: largest ever fraction of American population living in swing states?

[Mr. Morden]

I was just looking at some of the presidential vote margins in some of the larger states in the three close elections of the 1960s and '70s (1960; 1968; 1976).  I was struck by how close so many of the largest states were.  A stark contrast to today, when CA, TX, and NY are not at all in play in any close national election.  The most impressive election was 1976, when all six of the six most populous states had margins of 5% or less, and all eight of the eight most populous states had margins of 6% or less!

I was just wondering, would this make the 1960s/70s the point in time when the largest fraction of the American population lived in swing states (where swing states would be defined as the states that would be decided by about 5% or less in an election that was really close nationally)?  Or is there some other time in American history when it would have been higher?