There weren’t presidential debates in the way we think of them now prior to the advent of radio. The 1960 debates were the first actual presidential debates between the major general election candidates.

There were a small handful of primary debates prior to the 1960 presidential debate.

I’ve always known about the primary debates prior to 1960, but finding legitimate historical scholarship on them is difficult. The Commission on Presidential Debates has the best information that I believe is the most accurate.

This is a great question, and perhaps someone else has some access to historical scholarship on the earlier debates. I regret that JSTOR and the other commercial databases I subscribe to have nothing on these.