How long have there been presidential primary debates?

[Mr. Morden]

Just curious.  The modern presidential primary system for determining major party presidential nominations has only really existed since 1972, with the McGovern-Fraser reforms.  But over how much of that time has a staple of the primary campaigns been televised presidential primary debates?  Nowadays, these mostly air the 24 hour cable news networks and are largely ignored by the broadcast networks, but of course there was no CNN before 1980.

And were there any cases in which an incumbent president participated in a primary debate?  I'm pretty sure that Bush didn't debate Buchanan in 1992, but did Reagan ever debate Ford?  Or Carter debate Kennedy?

[Mr. Morden]

Oh, and here's a clip from a Bush vs. Reagan primary debate in 1980:

http://www.youtube.com/watch?v=edchtf9MS7g