Data used in this chart is available: Historical TV Ratings For Presidential Debates. The tabular data include information for each debate in a presidential election.
For this chart, I chose the largest gross audience for each contest. In the case of the Carter-Reagan contest, there was only one debate. Carter refused to appear with third party candidate John Anderson, so the first "debate" included only the two challengers.
The "percent households" is my measurement, not one used by the TV industry. I used the gross household data from Nielsen and the number of US households from the Census bureau. This possibly understates the influence of television in the Kennedy-Nixon debates, because the number of households without a television may have been a larger subset than in the later debates, which started in 1976 with Ford-Carter.